Why do ndb.Model subclasses expand when populated?

We have a model defined as ...

class Foo(ndb.Model): bar = ndb.TextProperty()

If I create or get, an instance Fooand call it Foo, it works ...

foo.populate(**{'SPAM': None})

This gives a Foonew property SPAM.

I would like to know how to make it throw an exception when updating a nonexistent property. I thought this should make it out of the box.

For writing, the new properties are also untyped, so I can do SPAMbe 5, and then "five", without any problems.