diff --git a/lib/assert/error.js b/lib/assert/error.js
index 1284017..1a583db 100644
--- a/lib/assert/error.js
+++ b/lib/assert/error.js
@@ -3,7 +3,11 @@ var inspect = require('../vows/console').inspect;
 
 require('assert').AssertionError.prototype.toString = function () {
     var that = this,
+        source;
+
+    if (this.stack) {
         source = this.stack.match(/([a-zA-Z0-9._-]+\.js)(:\d+):\d+/);
+    }
 
     function parse(str) {
         return str.replace(/{actual}/g,   inspect(that.actual)).
@@ -15,7 +19,7 @@ require('assert').AssertionError.prototype.toString = function () {
 
     if (this.message) {
         return stylize(parse(this.message), 'yellow') +
-               stylize(' // ' + source[1] + source[2], 'grey');
+               (source) ? stylize(' // ' + source[1] + source[2], 'grey') : '';
     } else {
         return stylize([
             this.expected,