When trying to implement Tag Max Value no SQL - #2 by pturmel I found that using key access with brackets i.e. state['max'] results in an Error_ExpressionEval. Fortunately, using state.get('max'). still works correctly.
EDIT: I'm guessing it's because it throws a KeyError, d'oh! I was thinking that maybe I could "pre-load" the key but I also bet that it resets the "state" dictionary whenever the expression is edited.