Glyphs.defaults is incompatible with Python `in`

From the docs:

# Check for whether or not a preference exists
if "" in Glyphs.defaults:
	# do stuff

But this is what happens (Glyphs 3.0.3 (3060)):

Why do you need that? That is not easily possible to verify. If there is a registered default, you can’t really discern if there is a user value or the default. I could add a __in__ method to the proxy that would just do return self[key] is not None.

I don’t need it; checking for None is sufficient in my use-case. It’s just the first thing I tried since it is the first thing the docs suggests doing.