NullPointerException in CategoryWrapper$1.getCollator

Hi,

I'm working on a plugin that uses three ScriptFunctionExtensions.
The report uses two of the functions in it's beforeFactory method and those two functions modify the report design by adding in a new scripted dataset which calls the third function.

The first two functions are both working, but the third causes a NullPointerException in BIRT before my code even runs.
In BIRT 4.2.2 the NPE is at:
CategoryWrapper$1.getCollator(Scriptable) line: 107

Object obj = scope.getPrototype( ).get( "compare_locale", scope.getPrototype( ) );

The issue being that the scope object for my function has no prototype.

Does anyone know what could cause that?

Thanks.

Jim