diff --git a/src/Collections/DeltaConfigCollection.php b/src/Collections/DeltaConfigCollection.php
index d381f37..dcbeb0d 100644
--- a/src/Collections/DeltaConfigCollection.php
+++ b/src/Collections/DeltaConfigCollection.php
@@ -117,7 +117,7 @@ public function getMiddlewares()
      */
     public function getDeltas($class)
     {
-        $classKey = strtolower($class);
+        $classKey = strtolower($class ?? '');
         return isset($this->deltas[$classKey])
             ? $this->deltas[$classKey]
             : [];
@@ -239,7 +239,7 @@ protected function clearDeltas($class = null, $key = null)
         }
 
         // Clear just one class
-        $classKey = strtolower($class);
+        $classKey = strtolower($class ?? '');
         if (!$key) {
             unset($this->deltas[$classKey]);
             return;
@@ -250,13 +250,13 @@ protected function clearDeltas($class = null, $key = null)
             return;
         }
         $this->deltas[$classKey] = array_filter(
-            $this->deltas[$classKey],
+            $this->deltas[$classKey] ?? [],
             function ($delta) use ($key) {
                 // Clear if an array with exactly one element, with the key
                 // being the affected config property
                 return !isset($delta['config'])
                     || !is_array($delta['config'])
-                    || (count($delta['config']) !== 1)
+                    || (count($delta['config'] ?? []) !== 1)
                     || !isset($delta['config'][$key]);
             }
         );
@@ -270,7 +270,7 @@ function ($delta) use ($key) {
      */
     protected function addDelta($class, $delta)
     {
-        $classKey = strtolower($class);
+        $classKey = strtolower($class ?? '');
         if (!isset($this->deltas[$classKey])) {
             $this->deltas[$classKey] = [];
         }
diff --git a/src/Collections/MemoryConfigCollection.php b/src/Collections/MemoryConfigCollection.php
index 6f56c45..82859f6 100644
--- a/src/Collections/MemoryConfigCollection.php
+++ b/src/Collections/MemoryConfigCollection.php
@@ -79,7 +79,7 @@ public function set($class, $name, $data, $metadata = [])
     {
         $this->saveMetadata($class, $metadata);
 
-        $classKey = strtolower($class);
+        $classKey = strtolower($class ?? '');
         if ($name) {
             if (!isset($this->config[$classKey])) {
                 $this->config[$classKey] = [];
@@ -122,7 +122,7 @@ public function get($class, $name = null, $excludeMiddleware = 0)
     protected function getClassConfig($class, $excludeMiddleware = 0)
     {
         // `true` excludes all middleware, so bypass call cache
-        $classKey = strtolower($class);
+        $classKey = strtolower($class ?? '');
         if ($excludeMiddleware === true) {
             return isset($this->config[$classKey]) ? $this->config[$classKey] : [];
         }
@@ -156,14 +156,14 @@ public function exists($class, $name = null, $excludeMiddleware = 0)
             return false;
         }
         if ($name) {
-            return array_key_exists($name, $config);
+            return array_key_exists($name, $config ?? []);
         }
         return true;
     }
 
     public function remove($class, $name = null)
     {
-        $classKey = strtolower($class);
+        $classKey = strtolower($class ?? '');
         if ($name) {
             unset($this->config[$classKey][$name]);
         } else {
@@ -248,7 +248,7 @@ protected function getSerializedMembers()
     {
         return array_filter(array_keys(get_object_vars($this)), function ($key) {
             // Skip $_underscoreProp
-            return strpos($key, '_') !== 0;
+            return strpos($key ?? '', '_') !== 0;
         });
     }
 
@@ -289,7 +289,7 @@ public function serialize()
      */
     public function unserialize($serialized)
     {
-        $data = unserialize($serialized);
+        $data = unserialize($serialized ?? '');
         $this->__unserialize($data);
     }
 
@@ -310,7 +310,7 @@ protected function saveMetadata($class, $metadata)
             return;
         }
 
-        $classKey = strtolower($class);
+        $classKey = strtolower($class ?? '');
         if (isset($this->metadata[$classKey]) && isset($this->config[$classKey])) {
             if (!isset($this->history[$classKey])) {
                 $this->history[$classKey] = [];
diff --git a/src/MergeStrategy/Priority.php b/src/MergeStrategy/Priority.php
index 163fe74..6557981 100644
--- a/src/MergeStrategy/Priority.php
+++ b/src/MergeStrategy/Priority.php
@@ -71,7 +71,7 @@ public static function mergeArray(array $highPriority, array $lowPriority)
             }
 
             // If not set, or we're changing type we can set low priority
-            if (is_int($key) || !array_key_exists($key, $lowPriority) || !is_array($lowPriority[$key])) {
+            if (is_int($key) || !array_key_exists($key, $lowPriority ?? []) || !is_array($lowPriority[$key])) {
                 if (is_int($key)) {
                     $lowPriority[] = $value;
                 } else {
diff --git a/src/Middleware/DeltaMiddleware.php b/src/Middleware/DeltaMiddleware.php
index 16b3283..cf5ddc8 100644
--- a/src/Middleware/DeltaMiddleware.php
+++ b/src/Middleware/DeltaMiddleware.php
@@ -100,7 +100,7 @@ protected function applyDelta($config, $delta)
             case DeltaConfigCollection::REPLACE:
                 return $delta['config'];
             case DeltaConfigCollection::REMOVE:
-                return array_diff_key($config, $delta['config']);
+                return array_diff_key($config ?? [], $delta['config']);
             default:
                 throw new InvalidArgumentException("Invalid delta " . $delta['type']);
         }
diff --git a/src/Middleware/MiddlewareAware.php b/src/Middleware/MiddlewareAware.php
index 8c4cb28..412b6bb 100644
--- a/src/Middleware/MiddlewareAware.php
+++ b/src/Middleware/MiddlewareAware.php
@@ -51,7 +51,7 @@ protected function callMiddleware($class, $excludeMiddleware, $last)
         $next = $last;
 
         /** @var Middleware $middleware */
-        foreach (array_reverse($this->getMiddlewares()) as $middleware) {
+        foreach (array_reverse($this->getMiddlewares() ?? []) as $middleware) {
             $next = function ($class, $excludeMiddleware) use ($middleware, $next) {
                 return $middleware->getClassConfig($class, $excludeMiddleware, $next);
             };
diff --git a/src/Middleware/MiddlewareCommon.php b/src/Middleware/MiddlewareCommon.php
index bd47e4c..b30bc7e 100644
--- a/src/Middleware/MiddlewareCommon.php
+++ b/src/Middleware/MiddlewareCommon.php
@@ -73,7 +73,7 @@ public function __unserialize(array $data): void
      */
     public function serialize()
     {
-        return json_encode(array_values($this->__serialize()));
+        return json_encode(array_values($this->__serialize() ?? []));
     }
 
     /**
@@ -86,9 +86,9 @@ public function serialize()
      */
     public function unserialize($serialized)
     {
-        $values = json_decode($serialized, true);
-        foreach (array_keys($this->__serialize()) as $i => $key) {
-            if (!property_exists($this, $key)) {
+        $values = json_decode($serialized ?? '', true);
+        foreach (array_keys($this->__serialize() ?? []) as $i => $key) {
+            if (!property_exists($this, $key ?? '')) {
                 continue;
             }
             $this->{$key} = $values[$i] ?? 0;
diff --git a/src/Transformer/PrivateStaticTransformer.php b/src/Transformer/PrivateStaticTransformer.php
index 5899a33..a9fa09e 100644
--- a/src/Transformer/PrivateStaticTransformer.php
+++ b/src/Transformer/PrivateStaticTransformer.php
@@ -40,7 +40,7 @@ public function transform(MutableConfigCollectionInterface $collection)
 
         foreach ($classes as $class) {
             // Skip if the class doesn't exist
-            if (!class_exists($class)) {
+            if (!class_exists($class ?? '')) {
                 continue;
             }
 
@@ -123,11 +123,11 @@ protected function isConfigProperty(ReflectionProperty $prop)
         }
         $annotations = $prop->getDocComment();
         // Whitelist @config
-        if (strstr($annotations, '@config')) {
+        if (strstr($annotations ?? '', '@config')) {
             return true;
         }
         // Don't treat @internal as config
-        if (strstr($annotations, '@internal')) {
+        if (strstr($annotations ?? '', '@internal')) {
             return false;
         }
         return true;
diff --git a/src/Transformer/YamlTransformer.php b/src/Transformer/YamlTransformer.php
index b64683d..10ec831 100644
--- a/src/Transformer/YamlTransformer.php
+++ b/src/Transformer/YamlTransformer.php
@@ -140,7 +140,7 @@ public function transform(MutableConfigCollectionInterface $collection)
      */
     public function addRule($rule, Closure $func)
     {
-        $rule = strtolower($rule);
+        $rule = strtolower($rule ?? '');
         $this->rules[$rule] = $func;
         return $this;
     }
@@ -154,7 +154,7 @@ public function addRule($rule, Closure $func)
      */
     protected function hasRule($rule)
     {
-        $rule = strtolower($rule);
+        $rule = strtolower($rule ?? '');
         return isset($this->rules[$rule]);
     }
 
@@ -168,7 +168,7 @@ protected function hasRule($rule)
      */
     public function ignoreRule($rule)
     {
-        $rule = strtolower($rule);
+        $rule = strtolower($rule ?? '');
         $this->ignoreRules[$rule] = $rule;
     }
 
@@ -181,7 +181,7 @@ public function ignoreRule($rule)
      */
     protected function isRuleIgnored($rule)
     {
-        $rule = strtolower($rule);
+        $rule = strtolower($rule ?? '');
 
         return isset($this->ignoreRules[$rule]);
     }
@@ -199,7 +199,7 @@ protected function getNamedYamlDocuments()
         $documents = [];
         foreach ($unnamed as $uniqueKey => $document) {
             $header = YamlParser::parse($document['header']) ?: [];
-            $header = array_change_key_case($header, CASE_LOWER);
+            $header = array_change_key_case($header ?? [], CASE_LOWER);
 
             $content = YamlParser::parse($document['content']);
 
@@ -248,7 +248,7 @@ protected function splitYamlDocuments()
 
         // We need to loop through each file and parse the yaml content
         foreach ($this->files as $file) {
-            $lines = file($file, FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
+            $lines = file($file ?? '', FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
 
             $firstLine = true;
             $context = 'content';
@@ -398,8 +398,8 @@ protected function getMatchingDocuments($pattern, $documents, $flag)
 
         // If the pattern starts with a hash, it means we're looking for a single document
         // named without the hash.
-        if (strpos($pattern, '#') === 0) {
-            $name = substr($pattern, 1);
+        if (strpos($pattern ?? '', '#') === 0) {
+            $name = substr($pattern ?? '', 1);
             if (isset($documents[$name])) {
                 return [$documents[$name]];
             }
@@ -410,14 +410,14 @@ protected function getMatchingDocuments($pattern, $documents, $flag)
         // and likewise for Before="*"
         if ($pattern === '*') {
             return array_filter(
-                $documents,
+                $documents ?? [],
                 function ($document) use ($flag) {
                     if (empty($document['header'][$flag])) {
                         return true;
                     }
                     $otherPatterns = $document['header'][$flag];
                     if (is_array($otherPatterns)) {
-                        return !in_array('*', $otherPatterns);
+                        return !in_array('*', $otherPatterns ?? []);
                     }
                     return $otherPatterns !== '*';
                 }
@@ -428,11 +428,11 @@ function ($document) use ($flag) {
         // and check their filename and maybe their document name, depending on the pattern.
         // We don't want to do any pattern matching after the first hash as the document name
         // is assumed to follow it.
-        $firstHash = strpos($pattern, '#');
+        $firstHash = strpos($pattern ?? '', '#');
         $documentName = false;
         if ($firstHash !== false) {
-            $documentName = substr($pattern, $firstHash + 1);
-            $pattern = substr($pattern, 0, $firstHash);
+            $documentName = substr($pattern ?? '', $firstHash + 1);
+            $pattern = substr($pattern ?? '', 0, $firstHash);
         }
 
         // Replace all `*` with `[^\.][a-zA-Z0-9\-_\/\.]+`, and quote other characters
@@ -440,9 +440,9 @@ function ($document) use ($flag) {
             '[^\.][a-zA-Z0-9\-_\/\.]+',
             array_map(
                 function ($part) {
-                    return preg_quote($part, '%');
+                    return preg_quote($part ?? '', '%');
                 },
-                explode('*', trim($pattern, '/\\'))
+                explode('*', trim($pattern ?? '', '/\\'))
             )
         ).'([./\\\\]|$)%';
 
@@ -451,7 +451,7 @@ function ($part) {
             // Ensure filename is relative
             $filename = $this->makeRelative($document['filename']);
 
-            if (preg_match($patternRegExp, $filename)) {
+            if (preg_match($patternRegExp ?? '', $filename ?? '')) {
                 if (!empty($documentName) && $documentName !== $document['header']['name']) {
                     // If we're looking for a specific document. If not found we can continue
                     continue;
@@ -475,12 +475,12 @@ function ($part) {
      */
     protected function makeRelative($filename)
     {
-        $dir = substr($filename, 0, strlen($this->baseDirectory));
+        $dir = substr($filename ?? '', 0, strlen($this->baseDirectory ?? ''));
         if ($dir == $this->baseDirectory) {
-            return trim(substr($filename, strlen($this->baseDirectory)), DIRECTORY_SEPARATOR);
+            return trim(substr($filename ?? '', strlen($this->baseDirectory ?? '')), DIRECTORY_SEPARATOR);
         }
 
-        return trim($filename, DIRECTORY_SEPARATOR);
+        return trim($filename ?? '', DIRECTORY_SEPARATOR);
     }
 
     /**
@@ -577,7 +577,7 @@ protected function testRules($header, $flag)
      */
     protected function testSingleRule($rule, $params, $flag = self::ONLY_FLAG)
     {
-        $rule = strtolower($rule);
+        $rule = strtolower($rule ?? '');
         if (!$this->hasRule($rule)) {
             throw new Exception(sprintf('Rule \'%s\' doesn\'t exist.', $rule));
         }
diff --git a/tests/Transformer/PrivateStaticTransformerTest.php b/tests/Transformer/PrivateStaticTransformerTest.php
index 1755add..09db7cc 100644
--- a/tests/Transformer/PrivateStaticTransformerTest.php
+++ b/tests/Transformer/PrivateStaticTransformerTest.php
@@ -84,7 +84,7 @@ public function testMerge()
     public function testInvalidClass()
     {
         $class = 'SomeNonExistentClass';
-        if (class_exists($class)) {
+        if (class_exists($class ?? '')) {
             $this->markTestSkipped($class . ' exists but the test expects it not to.');
         }
 
diff --git a/tests/Transformer/YamlTransformerTest.php b/tests/Transformer/YamlTransformerTest.php
index b8bd587..42606eb 100644
--- a/tests/Transformer/YamlTransformerTest.php
+++ b/tests/Transformer/YamlTransformerTest.php
@@ -65,8 +65,8 @@ protected function getConfigDirectory()
         $dir = $this->root->url().'/'.$this->directory;
 
         // Create the directory if it doesn't exist.
-        if (!is_dir($dir)) {
-            mkdir($dir);
+        if (!is_dir($dir ?? '')) {
+            mkdir($dir ?? '');
         }
 
         return $dir;
@@ -77,7 +77,7 @@ protected function getConfigDirectory()
      */
     public function testEmptyFileIgnored()
     {
-        file_put_contents($this->getFilePath('empty.yml'), '');
+        file_put_contents($this->getFilePath('empty.yml') ?? '', '');
 
         $collection = new MemoryConfigCollection;
         $transformer = new YamlTransformer(
@@ -107,7 +107,7 @@ public function testEmptyDocumentIgnored()
 third: thirdValue
 YAML;
 
-        file_put_contents($this->getFilePath('config2.yml'), $content);
+        file_put_contents($this->getFilePath('config2.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $transformer = new YamlTransformer(
@@ -137,7 +137,7 @@ public function testNoHeader()
 YAML;
 
         $file = $this->getFilePath('config.yml');
-        file_put_contents($file, $content);
+        file_put_contents($file ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $transformer = new YamlTransformer(
@@ -172,7 +172,7 @@ public function testNoNameStatement()
 ---
 Test: blah
 YAML;
-        file_put_contents($this->getFilePath('config.yml'), $content);
+        file_put_contents($this->getFilePath('config.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $transformer = new YamlTransformer(
@@ -196,7 +196,7 @@ public function testBeforeAndAfterStatements()
 ---
 test: 'blah'
 YAML;
-        file_put_contents($this->getFilePath('first.yml'), $content);
+        file_put_contents($this->getFilePath('first.yml') ?? '', $content);
 
         $content = <<<'YAML'
 ---
@@ -205,7 +205,7 @@ public function testBeforeAndAfterStatements()
 ---
 test: 'overwritten'
 YAML;
-        file_put_contents($this->getFilePath('second.yml'), $content);
+        file_put_contents($this->getFilePath('second.yml') ?? '', $content);
 
         $content = <<<'YAML'
 ---
@@ -214,7 +214,7 @@ public function testBeforeAndAfterStatements()
 ---
 test: 'set first'
 YAML;
-        file_put_contents($this->getFilePath('zzz.yml'), $content);
+        file_put_contents($this->getFilePath('zzz.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $transformer = new YamlTransformer(
@@ -244,7 +244,7 @@ public function testBeforeAfterStatementsByHash()
 ---
 test: 'overwritten'
 YAML;
-        file_put_contents($this->getFilePath('first.yml'), $content);
+        file_put_contents($this->getFilePath('first.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $transformer = new YamlTransformer(
@@ -268,7 +268,7 @@ public function testBeforeAfterStatementWithPath()
 test: 'test'
 YAML;
         mkdir($this->getConfigDirectory().'/test');
-        file_put_contents($this->getFilePath('test/config.yml'), $content);
+        file_put_contents($this->getFilePath('test/config.yml') ?? '', $content);
 
         $content = <<<'YAML'
 ---
@@ -278,7 +278,7 @@ public function testBeforeAfterStatementWithPath()
 test: 'should not overwrite'
 YAML;
         mkdir($this->getConfigDirectory().'/test2');
-        file_put_contents($this->getFilePath('test2/config.yml'), $content);
+        file_put_contents($this->getFilePath('test2/config.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $transformer = new YamlTransformer(
@@ -297,7 +297,7 @@ public function testBeforeAfterStatementWithPath()
 ---
 test: 'overwrite'
 YAML;
-        file_put_contents($this->getFilePath('test2/config.yml'), $content);
+        file_put_contents($this->getFilePath('test2/config.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $transformer = new YamlTransformer(
@@ -315,7 +315,7 @@ public function testBeforeAfterStatementWithPath()
 ---
 test: 'anything else'
 YAML;
-        file_put_contents($this->getFilePath('test2/config.yml'), $content);
+        file_put_contents($this->getFilePath('test2/config.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $transformer = new YamlTransformer(
@@ -333,7 +333,7 @@ public function testBeforeAfterStatementWithPath()
 ---
 test: 'another thing'
 YAML;
-        file_put_contents($this->getFilePath('test2/config.yml'), $content);
+        file_put_contents($this->getFilePath('test2/config.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $transformer = new YamlTransformer(
@@ -355,7 +355,7 @@ public function testBeforeAfterStatementWithNestedPath()
 YAML;
         mkdir($this->getConfigDirectory().'/test');
         mkdir($this->getConfigDirectory().'/test/test1-1');
-        file_put_contents($this->getFilePath('test/test1-1/config.yml'), $content);
+        file_put_contents($this->getFilePath('test/test1-1/config.yml') ?? '', $content);
 
         $content = <<<'YAML'
 ---
@@ -365,7 +365,7 @@ public function testBeforeAfterStatementWithNestedPath()
 test: 'should not overwrite'
 YAML;
         mkdir($this->getConfigDirectory().'/test2');
-        file_put_contents($this->getFilePath('test2/config.yml'), $content);
+        file_put_contents($this->getFilePath('test2/config.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $transformer = new YamlTransformer(
@@ -383,7 +383,7 @@ public function testBeforeAfterStatementWithNestedPath()
 ---
 test: 'overwrite'
 YAML;
-        file_put_contents($this->getFilePath('test2/config.yml'), $content);
+        file_put_contents($this->getFilePath('test2/config.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $transformer = new YamlTransformer(
@@ -401,7 +401,7 @@ public function testBeforeAfterStatementWithNestedPath()
 ---
 test: 'this will not overwrite'
 YAML;
-        file_put_contents($this->getFilePath('test2/config.yml'), $content);
+        file_put_contents($this->getFilePath('test2/config.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $transformer = new YamlTransformer(
@@ -419,7 +419,7 @@ public function testBeforeAfterStatementWithNestedPath()
 ---
 test: 'this will overwrite'
 YAML;
-        file_put_contents($this->getFilePath('test2/config.yml'), $content);
+        file_put_contents($this->getFilePath('test2/config.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $transformer = new YamlTransformer(
@@ -451,7 +451,7 @@ public function testCircularDependency()
 ---
 test: test
 YAML;
-        file_put_contents($this->getFilePath('config.yml'), $content);
+        file_put_contents($this->getFilePath('config.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $transformer = new YamlTransformer(
@@ -490,7 +490,7 @@ public function testSingleOnlyExceptStatements()
 ---
 test: 'not applied'
 YAML;
-        file_put_contents($this->getFilePath('config.yml'), $content);
+        file_put_contents($this->getFilePath('config.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $yaml = new YamlTransformer(
@@ -537,7 +537,7 @@ public function testMultipleOnlyExceptStatements()
 ---
 test: 'not applied'
 YAML;
-        file_put_contents($this->getFilePath('config.yml'), $content);
+        file_put_contents($this->getFilePath('config.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $yaml = new YamlTransformer(
@@ -647,7 +647,7 @@ public function testFailedOnlyExceptStatements()
 test2: 'test2-only-error'
 
 YAML;
-        file_put_contents($this->getFilePath('config.yml'), $content);
+        file_put_contents($this->getFilePath('config.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $yaml = new YamlTransformer(
@@ -687,7 +687,7 @@ public function testIgnoredOnlyExceptRule()
 ---
 test2: 'test2'
 YAML;
-        file_put_contents($this->getFilePath('config.yml'), $content);
+        file_put_contents($this->getFilePath('config.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $yaml = new YamlTransformer(
@@ -740,7 +740,7 @@ public function testKeyValueOnlyExceptStatements()
 ---
 arrays: passed
 YAML;
-        file_put_contents($this->getFilePath('config.yml'), $content);
+        file_put_contents($this->getFilePath('config.yml') ?? '', $content);
 
         $collection = new MemoryConfigCollection;
         $yaml = new YamlTransformer(