diff --git a/lib/ace/lib/regexp.js b/lib/ace/lib/regexp.js
index 09e2bb7e..c3b9f5b5 100644
--- a/lib/ace/lib/regexp.js
+++ b/lib/ace/lib/regexp.js
@@ -42,7 +42,7 @@ define(function(require, exports, module) {
     RegExp.prototype.exec = function (str) {
         var match = real.exec.apply(this, arguments),
             name, r2;
-        if (match) {
+        if ( typeof(str) == 'string' && match) {
             // Fix browsers whose `exec` methods don't consistently return `undefined` for
             // nonparticipating capturing groups
             if (!compliantExecNpcg && match.length > 1 && indexOf(match, "") > -1) {
@@ -107,4 +107,4 @@ define(function(require, exports, module) {
         return -1;
     };
 
-});
\ No newline at end of file
+});