--- a/testing/mochitest/browser-harness.xul
+++ b/testing/mochitest/browser-harness.xul
@@ -196,16 +196,18 @@
         }
         return html;
       }
     };
 
     // Returns an array of browserTest objects for all the selected tests
     function listTests() {
       [links, singleTestPath] = getTestList();
+      if (!links)
+        return [];
 
       // load server.js in so we can share template functions
       var scriptLoader = Cc["@mozilla.org/moz/jssubscript-loader;1"].
                            getService(Ci.mozIJSSubScriptLoader);
       var srvScope = {};
       scriptLoader.loadSubScript('chrome://mochikit/content/server.js',
                                  srvScope);
 

--- a/testing/mochitest/chrome-harness.js
+++ b/testing/mochitest/chrome-harness.js
@@ -128,16 +128,19 @@ function getMochitestJarListing(aBasePat
         var singleObject = {};
         singleObject[singleTestPath] = true;
         return [singleObject, singleTestPath];
       }
     }
     else if (zReader.hasEntry(pathToCheck + "/")) {
       base = pathToCheck + "/";
     }
+    else {
+      return [];
+    }
   }
   var [links, count] = zList(base, zReader, basePath, true);
   return [links, null];
 }
 
 /*
  * Replicate the server.js list() function with a .jar file
  *