diff --git a/src/TestCase/TestDoubles.php b/src/TestCase/TestDoubles.php
index c04fe9e..08f0088 100644
--- a/src/TestCase/TestDoubles.php
+++ b/src/TestCase/TestDoubles.php
@@ -15,7 +15,7 @@ trait TestDoubles
 {
     abstract public function getMockBuilder(string $className): MockBuilder;
 
-    abstract protected function prophesize($classOrInterface = null): ObjectProphecy;
+    abstract protected function prophesize(?string $classOrInterface = null): ObjectProphecy;
 
     /**
      * @before