I assumed that the parser would be called on to do its thing within the scope of the current method.

In which case, it would work, and I'm talking out my lower lumbar region again :)

Needless to say, I assumed that it wouldn't be called until later, which was the case when I 'discovered' this problem, and after falling foul of trying to push and pop instance handles for multiple instances, arrived at the work-around of passing a ref to a closure around a call to the method.