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free module changes dimensions after base change

Hi, I have a free module of rank 1 over the integers

sage: M

Free module of degree 1 and rank 1 over Integer Ring
Echelon basis matrix:
[2]

However, when I base change it to the field with two elements, it changes dimension!

sage: M.change_ring(GF(2))

Vector space of degree 1 and dimension 0 over Finite Field of size 2
Basis matrix:
[]

What's going on?

P.S.: What does Sage mean by "degree"?