diff -r a6a23aa7a546 -r c6f21b56a6cf vm/src/main/java/org/apidesign/vm4brwsr/ByteCodeToJavaScript.java
--- a/vm/src/main/java/org/apidesign/vm4brwsr/ByteCodeToJavaScript.java	Thu Dec 06 16:11:48 2012 +0100
+++ b/vm/src/main/java/org/apidesign/vm4brwsr/ByteCodeToJavaScript.java	Sun Dec 09 16:43:36 2012 +0100
@@ -759,9 +759,9 @@
                     final String type = jc.getClassName(indx);
                     if (!type.startsWith("[")) {
                         // no way to check arrays right now
-                        out.append("if(s[s.length - 1].$instOf_")
+                        out.append("if(s[s.length - 1] !== null && !s[s.length - 1].$instOf_")
                            .append(type.replace('/', '_'))
-                           .append(" != 1) throw {};"); // XXX proper exception
+                           .append(") throw {};"); // XXX proper exception
                     }
                     i += 2;
                     break;