diff -r bf08bd96d408 -r b66d6e7756bb rt/vm/src/main/java/org/apidesign/vm4brwsr/VM.java
--- a/rt/vm/src/main/java/org/apidesign/vm4brwsr/VM.java	Thu May 22 19:06:44 2014 +0200
+++ b/rt/vm/src/main/java/org/apidesign/vm4brwsr/VM.java	Fri May 23 11:44:38 2014 +0200
@@ -526,8 +526,12 @@
                 + "        var at = args[i];\n"
                 + "        var ret;\n"
                 + "        if (typeof at === 'function') ret = at(name, skip);\n"
-                + "        else ret = vm['org_apidesign_vm4brwsr_ClassPath'](false).\n"
-                + "          loadBytes___3BLjava_lang_String_2Ljava_lang_Object_2II(name, args, i, skip);\n"
+                + "        else {\n"
+                + "          var cp = vm['org_apidesign_vm4brwsr_ClassPath'];\n"
+                + "          if (!cp) throw 'Core Java library not registered. Cannot load from ' + at;\n"
+                + "          ret = cp(false).\n"
+                + "            loadBytes___3BLjava_lang_String_2Ljava_lang_Object_2II(name, args, i, skip);\n"
+                + "        }\n"
                 + "        if (ret !== null) return ret;\n"
                 + "      }\n"
                 + "      while (knownExtensions < extensions.length) {\n"