diff --git a/src/AsyncTestCase.php b/src/AsyncTestCase.php
index 38a6076..38bfc5d 100644
--- a/src/AsyncTestCase.php
+++ b/src/AsyncTestCase.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\PHPUnit;
 
diff --git a/src/CallbackStub.php b/src/CallbackStub.php
index 0028693..83c6a0b 100644
--- a/src/CallbackStub.php
+++ b/src/CallbackStub.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\PHPUnit;
 
diff --git a/src/TestException.php b/src/TestException.php
index dc356fd..ffb1db5 100644
--- a/src/TestException.php
+++ b/src/TestException.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\PHPUnit;
 
diff --git a/src/UnhandledException.php b/src/UnhandledException.php
index da0ee85..a2d6471 100644
--- a/src/UnhandledException.php
+++ b/src/UnhandledException.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\PHPUnit;
 
diff --git a/test/AsyncTestCaseTest.php b/test/AsyncTestCaseTest.php
index 4ad88d6..a8f39b7 100644
--- a/test/AsyncTestCaseTest.php
+++ b/test/AsyncTestCaseTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\PHPUnit\Test;
 
diff --git a/test/InvalidAsyncTestCaseTest.php b/test/InvalidAsyncTestCaseTest.php
index 82ce528..ee48f5a 100644
--- a/test/InvalidAsyncTestCaseTest.php
+++ b/test/InvalidAsyncTestCaseTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\PHPUnit\Test;