diff --git a/lib/adapters/xhr.js b/lib/adapters/xhr.js
index a386dd2..48d2e8e 100644
--- a/lib/adapters/xhr.js
+++ b/lib/adapters/xhr.js
@@ -104,7 +104,7 @@ module.exports = function xhrAdapter(config) {
 
     // Handle timeout
     request.ontimeout = function handleTimeout() {
-      var timeoutErrorMessage = 'timeout of ' + config.timeout + 'ms exceeded';
+      var timeoutErrorMessage = config.timeout ? 'timeout of ' + config.timeout + 'ms exceeded' : 'timeout exceeded';
       if (config.timeoutErrorMessage) {
         timeoutErrorMessage = config.timeoutErrorMessage;
       }