diff --git a/lib/mockResponse.js b/lib/mockResponse.js
index e244aee..bd4d60c 100644
--- a/lib/mockResponse.js
+++ b/lib/mockResponse.js
@@ -110,7 +110,7 @@ function createResponse(options) {
     mockResponse.writeHead = function(statusCode, statusMessage, headers) {
 
         if (_endCalled) {
-            throw 'The end() method has already been called.';
+            throw new Error('The end() method has already been called.');
         }
 
         if (mockResponse.headersSent) {