diff --git a/src/TestUtils/WorkshopExerciseTest.php b/src/TestUtils/WorkshopExerciseTest.php
index 69de5829..7ceb12db 100644
--- a/src/TestUtils/WorkshopExerciseTest.php
+++ b/src/TestUtils/WorkshopExerciseTest.php
@@ -60,7 +60,7 @@ private function getExercise(): ExerciseInterface
             ->findByClassName($this->getExerciseClass());
     }
 
-    public function runExercise(string $submissionFile)
+    public function runExercise(string $submissionFile): void
     {
         $exercise = $this->getExercise();