There is a "reason" (maybe not really an *excuse*)..
My original implementation was using numpy recarrays (i.e. typed tuple values).
This was so that I could do more general lookup operations with "np.where()".  As it turned out, I only ever used one lookup method, so I simplified it back down to a dictionary with specific parameters in the key for the lookup.

Given the recent discussion about maybe needing enhanced lookup strategies, this issue could even come back.

But for now, I think it *could* use a None, which would be more natural.