diff --git a/src/Illuminate/Testing/TestResponse.php b/src/Illuminate/Testing/TestResponse.php
index ea19db479a03..ebf06a7dc1fd 100644
--- a/src/Illuminate/Testing/TestResponse.php
+++ b/src/Illuminate/Testing/TestResponse.php
@@ -1651,7 +1651,7 @@ public function transformNotSuccessfulException($exception)
      */
     protected function appendExceptionToException($exceptionToAppend, $exception)
     {
-        $exceptionMessage = $exceptionToAppend->getMessage();
+        $exceptionMessage = is_string($exceptionToAppend) ? $exceptionToAppend : $exceptionToAppend->getMessage();
 
         $exceptionToAppend = (string) $exceptionToAppend;