diff --git a/lib/ace/worker/worker.js b/lib/ace/worker/worker.js
index faf5624e..e8bf0d3e 100644
--- a/lib/ace/worker/worker.js
+++ b/lib/ace/worker/worker.js
@@ -135,7 +135,10 @@ var sender;
 onmessage = function(e) {
     var msg = e.data;
     if (msg.command) {
-        main[msg.command].apply(main, msg.args);
+        if (main[msg.command])
+            main[msg.command].apply(main, msg.args);
+        else
+            throw new Error("Unknown command:" + msg.command);
     }
     else if (msg.init) {        
         initBaseUrls(msg.tlns);