diff -r 885acca2fa0b -r 39695cc9a54f vm/src/main/java/org/apidesign/vm4brwsr/VM.java
--- a/vm/src/main/java/org/apidesign/vm4brwsr/VM.java	Tue Dec 11 09:36:44 2012 +0100
+++ b/vm/src/main/java/org/apidesign/vm4brwsr/VM.java	Tue Dec 11 09:48:37 2012 +0100
@@ -115,6 +115,7 @@
             + "      var attr = name.replace__Ljava_lang_String_2CC(name, '.','_');\n"
             + "      var fn = vm[attr];\n"
             + "      if (fn) return fn(false);\n"
+            + "      if (!args[0]) throw 'bck2brwsr initialized without loader function, cannot load ' + name;\n"
             + "      return vm.org_apidesign_vm4brwsr_VMLazy(false).\n"
             + "        load___3Ljava_lang_Object_2Ljava_lang_Object_2Ljava_lang_String_2_3Ljava_lang_Object_2(loader, name, args);\n"
             + "    }\n"