Some Python code seems to have allow_none but I didn't see any simple way to pass allow_none through Frontier::Client. The source code made some references to the use of None but it seem to be just about default fonts.
It has been suggested that the type mismatch happens where "None" is used instead of "False". I don't see exactly where this might happen but maybe you'll be able to identify something with in debug mode (ie, passing the debug switch to Frontier::Client->new).
Of course, you still not be able to fix it without changing source code so it might even be easier to modify Frontier::Client to pass allow_none, exactly like it does the other options. Could make a feature request -- this module was last updated 10 years ago but the author could be listening.