diff --git a/.travis.yml b/.travis.yml
index 12b6dd49..d500b9d6 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,18 +1,14 @@
 language: php
 
-php:
-  - 7.1
-  - 7.2
-  - 7.3
-  - 7.4
-  - nightly # ignore errors, see below
-
 # lock distro so new future defaults will not break the build
 dist: trusty
 
-matrix:
-  allow_failures:
-    - php: nightly
+jobs:
+  include:
+    - php: 7.1
+    - php: 7.2
+    - php: 7.3
+    - php: 7.4
 
 install:
   - composer install
diff --git a/composer.json b/composer.json
index 278b1fef..76e286aa 100644
--- a/composer.json
+++ b/composer.json
@@ -9,7 +9,7 @@
         "php": ">=7.1.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^7"
+        "phpunit/phpunit": "^9.0 || ^7.5"
     },
     "autoload": {
         "psr-4": {
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index 3d184322..0b97c746 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -1,15 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
-<phpunit backupGlobals="false"
-         backupStaticAttributes="false"
-         colors="true"
-         convertErrorsToExceptions="true"
-         convertNoticesToExceptions="true"
-         convertWarningsToExceptions="true"
-         processIsolation="false"
-         stopOnFailure="false"
-         bootstrap="vendor/autoload.php"
->
+<phpunit bootstrap="vendor/autoload.php" colors="true">
     <testsuites>
         <testsuite name="Promise Test Suite">
             <directory>./tests/</directory>
diff --git a/tests/PromiseTest/PromiseFulfilledTestTrait.php b/tests/PromiseTest/PromiseFulfilledTestTrait.php
index 9213375c..0b79a95b 100644
--- a/tests/PromiseTest/PromiseFulfilledTestTrait.php
+++ b/tests/PromiseTest/PromiseFulfilledTestTrait.php
@@ -237,7 +237,7 @@ public function doneShouldTriggerFatalErrorThrownFulfillmentHandlerForFulfilledP
         $errors = $errorCollector->stop();
 
         self::assertEquals(E_USER_ERROR, $errors[0]['errno']);
-        self::assertContains('Unhandled Rejection', $errors[0]['errstr']);
+        self::assertStringContainsString('Unhandled Rejection', $errors[0]['errstr']);
     }
 
     /** @test */
@@ -257,7 +257,7 @@ public function doneShouldTriggerFatalErrorUnhandledRejectionExceptionWhenFulfil
         $errors = $errorCollector->stop();
 
         self::assertEquals(E_USER_ERROR, $errors[0]['errno']);
-        self::assertContains('Unhandled Rejection', $errors[0]['errstr']);
+        self::assertStringContainsString('Unhandled Rejection', $errors[0]['errstr']);
     }
 
     /** @test */
diff --git a/tests/PromiseTest/PromiseRejectedTestTrait.php b/tests/PromiseTest/PromiseRejectedTestTrait.php
index d2fb8ac0..68b401c4 100644
--- a/tests/PromiseTest/PromiseRejectedTestTrait.php
+++ b/tests/PromiseTest/PromiseRejectedTestTrait.php
@@ -229,7 +229,7 @@ public function doneShouldTriggerFatalErrorExceptionThrownByRejectionHandlerForR
         $errors = $errorCollector->stop();
 
         self::assertEquals(E_USER_ERROR, $errors[0]['errno']);
-        self::assertContains('Unhandled Rejection', $errors[0]['errstr']);
+        self::assertStringContainsString('Unhandled Rejection', $errors[0]['errstr']);
     }
 
     /** @test */
@@ -248,7 +248,7 @@ public function doneShouldTriggerFatalErrorRejectionExceptionWhenRejectionHandle
         $errors = $errorCollector->stop();
 
         self::assertEquals(E_USER_ERROR, $errors[0]['errno']);
-        self::assertContains('Unhandled Rejection', $errors[0]['errstr']);
+        self::assertStringContainsString('Unhandled Rejection', $errors[0]['errstr']);
     }
 
     /** @test */
diff --git a/tests/PromiseTest/RejectTestTrait.php b/tests/PromiseTest/RejectTestTrait.php
index 949426cd..c089e0ad 100644
--- a/tests/PromiseTest/RejectTestTrait.php
+++ b/tests/PromiseTest/RejectTestTrait.php
@@ -142,7 +142,7 @@ public function doneShouldTriggerFatalErrorExceptionThrownByRejectionHandler()
         $errors = $errorCollector->stop();
 
         self::assertEquals(E_USER_ERROR, $errors[0]['errno']);
-        self::assertContains('Unhandled Rejection', $errors[0]['errstr']);
+        self::assertStringContainsString('Unhandled Rejection', $errors[0]['errstr']);
     }
 
     /** @test */
@@ -161,7 +161,7 @@ public function doneShouldTriggerFatalErrorRejectionExceptionWhenRejectionHandle
         $errors = $errorCollector->stop();
 
         self::assertEquals(E_USER_ERROR, $errors[0]['errno']);
-        self::assertContains('Unhandled Rejection', $errors[0]['errstr']);
+        self::assertStringContainsString('Unhandled Rejection', $errors[0]['errstr']);
     }
 
     /** @test */
@@ -184,7 +184,7 @@ public function doneShouldTriggerFatalErrorUnhandledRejectionExceptionWhenReject
         $errors = $errorCollector->stop();
 
         self::assertEquals(E_USER_ERROR, $errors[0]['errno']);
-        self::assertContains('Unhandled Rejection', $errors[0]['errstr']);
+        self::assertStringContainsString('Unhandled Rejection', $errors[0]['errstr']);
     }
 
     /** @test */
@@ -201,7 +201,7 @@ public function doneShouldTriggerFatalErrorExceptionProvidedAsRejectionValue()
         $errors = $errorCollector->stop();
 
         self::assertEquals(E_USER_ERROR, $errors[0]['errno']);
-        self::assertContains('Unhandled Rejection', $errors[0]['errstr']);
+        self::assertStringContainsString('Unhandled Rejection', $errors[0]['errstr']);
     }
 
     /** @test */
diff --git a/tests/PromiseTest/ResolveTestTrait.php b/tests/PromiseTest/ResolveTestTrait.php
index d4694344..23105bfd 100644
--- a/tests/PromiseTest/ResolveTestTrait.php
+++ b/tests/PromiseTest/ResolveTestTrait.php
@@ -200,7 +200,7 @@ public function doneShouldTriggerFatalErrorExceptionThrownFulfillmentHandler()
         $errors = $errorCollector->stop();
 
         self::assertEquals(E_USER_ERROR, $errors[0]['errno']);
-        self::assertContains('Unhandled Rejection', $errors[0]['errstr']);
+        self::assertStringContainsString('Unhandled Rejection', $errors[0]['errstr']);
     }
 
     /** @test */
@@ -219,7 +219,7 @@ public function doneShouldTriggerFatalErrorUnhandledRejectionExceptionWhenFulfil
         $errors = $errorCollector->stop();
 
         self::assertEquals(E_USER_ERROR, $errors[0]['errno']);
-        self::assertContains('Unhandled Rejection', $errors[0]['errstr']);
+        self::assertStringContainsString('Unhandled Rejection', $errors[0]['errstr']);
     }
 
     /** @test */
diff --git a/tests/Stub/CallableStub.php b/tests/Stub/CallableStub.php
deleted file mode 100644
index 01208933..00000000
--- a/tests/Stub/CallableStub.php
+++ /dev/null
@@ -1,10 +0,0 @@
-<?php
-
-namespace React\Promise\Stub;
-
-class CallableStub
-{
-    public function __invoke()
-    {
-    }
-}
diff --git a/tests/TestCase.php b/tests/TestCase.php
index 50f701a0..8983571c 100644
--- a/tests/TestCase.php
+++ b/tests/TestCase.php
@@ -37,10 +37,14 @@ public function expectCallableNever(): callable
         return $mock;
     }
 
-    public function createCallableMock()
+    protected function createCallableMock()
     {
-        return $this
-            ->getMockBuilder(CallableStub::class)
-            ->getMock();
+        if (method_exists('PHPUnit\Framework\MockObject\MockBuilder', 'addMethods')) {
+            // PHPUnit 9+
+            return $this->getMockBuilder('stdClass')->addMethods(array('__invoke'))->getMock();
+        } else {
+            // legacy PHPUnit 4 - PHPUnit 9
+            return $this->getMockBuilder('stdClass')->setMethods(array('__invoke'))->getMock();
+        }
     }
 }