diff --git a/lib/XMLHttpRequest.js b/lib/XMLHttpRequest.js
index bada957..0f819f6 100644
--- a/lib/XMLHttpRequest.js
+++ b/lib/XMLHttpRequest.js
@@ -236,10 +236,12 @@ exports.XMLHttpRequest = function() {
     }
     var result = "";
 
-    for (var i in response.headers) {
+    var headers = this.response.headers;
+
+    for (var i in headers) {
       // Cookie headers are excluded
       if (i !== "set-cookie" && i !== "set-cookie2") {
-        result += i + ": " + response.headers[i] + "\r\n";
+        result += i + ": " + headers[i] + "\r\n";
       }
     }
     return result.substr(0, result.length - 2);