What does Field.vdot() do if the spaces keyword is present?

What is the expected result if we call

a.vdot(b,spaces=0)

where a is a Field living on one space and b is a Field living on two spaces?

I'm asking because the documentation says that the result should be float or complex, but the code actually returns a Field object.

Is there an undocumented constraint that both Fields must have the same number of domains?