diff --git a/.travis.yml b/.travis.yml
index f551481d..fe9f2614 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,10 +12,10 @@ matrix:
     - php: 7.1
     - php: 7.2
     - php: 7.3
-    - php: 7.3.0
-    - php: 7.3.1
-    - php: 7.3.2
+    - php: 7.4
     - php: nightly
+      env:
+        - IGNORE_PLATFORM_REQ=true
     - php: hhvm-3.9
       sudo: required
       dist: trusty
@@ -36,7 +36,12 @@ before_install:
   - xdebug-disable
 
 before_script:
-  - travis_retry composer update --no-interaction --prefer-source --prefer-stable
+  - if [[ "$IGNORE_PLATFORM_REQ" == "true" ]]; then
+      composer self-update --preview;
+      travis_retry composer update --no-interaction --prefer-stable --ignore-platform-req=php;
+    else
+      travis_retry composer update --no-interaction --prefer-stable;
+    fi
 
 script:
   - if [[ "$LINT" == "true" ]]; then
diff --git a/composer.json b/composer.json
index 619b8e27..cc326282 100644
--- a/composer.json
+++ b/composer.json
@@ -23,7 +23,7 @@
     "sort-packages": true
   },
   "require": {
-    "php": "^7",
+    "php": "^7.0 || ^8.0",
     "ext-intl": "*",
     "ext-simplexml": "*",
     "ext-mbstring": "*",
diff --git a/tests/Assert/Tests/AssertTest.php b/tests/Assert/Tests/AssertTest.php
index 8f4d3fea..317ac166 100644
--- a/tests/Assert/Tests/AssertTest.php
+++ b/tests/Assert/Tests/AssertTest.php
@@ -16,6 +16,7 @@
 
 use Assert\Assertion;
 use Assert\AssertionFailedException;
+use Assert\Tests\Fixtures\CustomAssertion;
 use Assert\Tests\Fixtures\OneCountable;
 use PDO;
 use PHPUnit\Framework\TestCase;
@@ -1832,7 +1833,8 @@ public function testValidCallable()
     {
         $this->assertTrue(Assertion::isCallable('\is_callable'));
         $this->assertTrue(Assertion::isCallable(__NAMESPACE__.'\\Fixtures\\someCallable'));
-        $this->assertTrue(Assertion::isCallable([OneCountable::class, 'count']));
+        $this->assertTrue(Assertion::isCallable([new OneCountable(), 'count']));
+        $this->assertTrue(Assertion::isCallable([CustomAssertion::class, 'clearCalls']));
         $this->assertTrue(
             Assertion::isCallable(
                 function () {
@@ -2227,7 +2229,7 @@ public function testInvalidPropertiesExist($properties)
 
     public function testIsResource()
     {
-        self::assertTrue(Assertion::isResource(\curl_init()));
+        self::assertTrue(Assertion::isResource(\fopen('php://memory', 'w')));
     }
 
     /**