Error: illegal operation on JavaArray prototype object

I get various errors like the subject line when testing objects
coming out of jsd with lines like:

return "length" in object && typeof(object.length) == "number";

or

if ( (typeof(object) != "undefined") && (typeof(object.isValid) != 
"undefined"))

The latter was an attempt to see if the object might be an
invalid frame.

I'm not doing anything with JavaObject or JavaArray that I
know about, but then the page could use them somehow.

Any hints?  Is there a test for JavaXXX that will not
fail with illegalOpertion?

Thanks,
John.