diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index caf694a..52c43d8 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -12,4 +12,6 @@ require_once(__DIR__ . '/../vendor/autoload.php');
 require_once(__DIR__ . '/../vendor/yiisoft/yii2/Yii.php');
 
 Yii::setAlias('@yiiunit/extensions/swiftmailer', __DIR__);
-Yii::setAlias('@yii/swiftmailer', dirname(__DIR__));
\ No newline at end of file
+Yii::setAlias('@yii/swiftmailer', dirname(__DIR__));
+
+require_once(__DIR__ . '/compatibility.php');
\ No newline at end of file
diff --git a/tests/compatibility.php b/tests/compatibility.php
new file mode 100644
index 0000000..9141b4f
--- /dev/null
+++ b/tests/compatibility.php
@@ -0,0 +1,12 @@
+<?php
+/*
+ * Ensures compatibility with PHPUnit 6.x
+ */
+
+if (!class_exists('PHPUnit_Framework_Constraint') && class_exists('PHPUnit\Framework\Constraint\Constraint')) {
+    abstract class PHPUnit_Framework_Constraint extends \PHPUnit\Framework\Constraint\Constraint {}
+}
+
+if (!class_exists('PHPUnit_Framework_TestCase') && class_exists('PHPUnit\Framework\TestCase')) {
+    abstract class PHPUnit_Framework_TestCase extends \PHPUnit\Framework\TestCase {}
+}