diff --git a/lib/Service/AbstractService.php b/lib/Service/AbstractService.php
index abcc13ae2..2cc759c07 100644
--- a/lib/Service/AbstractService.php
+++ b/lib/Service/AbstractService.php
@@ -43,18 +43,15 @@ public function getClient()
     private static function formatParams($params)
     {
         if (null === $params) {
-            return $params;
+            return null;
         }
-        $formatted = [];
-        foreach ($params as $k => $v) {
-            if (null === $v) {
-                $formatted[$k] = '';
-            } else {
-                $formatted[$k] = $v;
+        \array_walk_recursive($params, function (&$value, $key) {
+            if (null === $value) {
+                $value = '';
             }
-        }
+        });
 
-        return $formatted;
+        return $params;
     }
 
     protected function request($method, $path, $params, $opts)
diff --git a/tests/Stripe/Service/AbstractServiceTest.php b/tests/Stripe/Service/AbstractServiceTest.php
index deee3e386..cf79e063d 100644
--- a/tests/Stripe/Service/AbstractServiceTest.php
+++ b/tests/Stripe/Service/AbstractServiceTest.php
@@ -16,6 +16,9 @@ final class AbstractServiceTest extends \PHPUnit\Framework\TestCase
     /** @var CouponService */
     private $service;
 
+    /** @var \ReflectionMethod */
+    private $formatParamsReflector;
+
     /**
      * @before
      */
@@ -26,6 +29,15 @@ public function setUpMockService()
         $this->service = new \Stripe\Service\CouponService($this->client);
     }
 
+    /**
+     * @before
+     */
+    public function setUpReflectors()
+    {
+        $this->formatParamsReflector = new \ReflectionMethod(\Stripe\Service\AbstractService::class, 'formatParams');
+        $this->formatParamsReflector->setAccessible(true);
+    }
+
     public function testNullGetsEmptyStringified()
     {
         $this->expectException(\Stripe\Exception\InvalidRequestException::class);
@@ -57,4 +69,26 @@ public function testRetrieveThrowsIfIdNullIsWhitespace()
 
         $this->service->retrieve(' ');
     }
+
+    public function testFormatParams()
+    {
+        $result = $this->formatParamsReflector->invoke(null, ['foo' => null]);
+        static::assertTrue('' === $result['foo']);
+        static::assertTrue(null !== $result['foo']);
+
+        $result = $this->formatParamsReflector->invoke(null, ['foo' => ['bar' => null, 'baz' => 1, 'nest' => ['triplynestednull' => null, 'triplynestednonnull' => 1]]]);
+        static::assertTrue('' === $result['foo']['bar']);
+        static::assertTrue(null !== $result['foo']['bar']);
+        static::assertTrue(1 === $result['foo']['baz']);
+        static::assertTrue('' === $result['foo']['nest']['triplynestednull']);
+        static::assertTrue(1 === $result['foo']['nest']['triplynestednonnull']);
+
+        $result = $this->formatParamsReflector->invoke(null, ['foo' => ['zero', null, null, 'three'], 'toplevelnull' => null, 'toplevelnonnull' => 4]);
+        static::assertTrue('zero' === $result['foo'][0]);
+        static::assertTrue('' === $result['foo'][1]);
+        static::assertTrue('' === $result['foo'][2]);
+        static::assertTrue('three' === $result['foo'][3]);
+        static::assertTrue('' === $result['toplevelnull']);
+        static::assertTrue(4 === $result['toplevelnonnull']);
+    }
 }