diff --git a/src/PHPUnit/PHPMatcherConstraint.php b/src/PHPUnit/PHPMatcherConstraint.php
index cc231768..ce55d504 100644
--- a/src/PHPUnit/PHPMatcherConstraint.php
+++ b/src/PHPUnit/PHPMatcherConstraint.php
@@ -27,7 +27,7 @@ public function toString() : string
         return 'matches the pattern';
     }
 
-    protected function additionalFailureDescription($other)
+    protected function additionalFailureDescription($other) : string
     {
         return $this->matcher->getError();
     }