As reported by Tal Liron at http://mail.openjdk.java.net/pipermail/nashorn-dev/2014-August/003307.html The following program will cause an exception when "isEmpty2" is called: Error: java.lang.ClassCastException: java.lang.String cannot be cast to jdk.nashorn.internal.runtime.Undefined The only difference between isEmpty1 and isEmpty2 is the actual use of "k". If "k" is not referenced, the exception happens. function isEmpty1(v) { for (var k in v) { print(v[k]+'\n') return false } return true } function isEmpty2(v) { for (var k in v) { return false } return true } x = {test: 'test'} isEmpty1(x) isEmpty2(x)