Does it give you a stack trace for the null pointer exception pointing out the rogue class?
My bet would be on one of the custom application attributes you have put into scope (ie those without a package/qualified namespace)
Could the toString() method of any of these objects produce a null pointer exception?
entry is not likely null at any time. But the default action is to emit the string equivalents of the key and value. So as Stephan pointed out, the toString()'s of any of the entry keys or values can cause this issue if they NPPE.