Have you heard the one about the

newfie and the polack? (edited)

Some "jokes" are just not that funny.

Canadians do tend to be a bit sensitive about being assumed to be Americans, because we aren't! It is a sore point; which is why you got the reaction you did. But part of being a good friend and neighbour includes not poking your friend's sore spots.

Your "joke" was belittling and insulting. I am prepared to give you the benefit of the doubt and assume you didn't mean it that way (or you didn't think about the differences between our cultures before you typed), but be assured that that is how it came across. When interacting across cultural barriers, it is quite possible to give unintentional offense (like eating with your left hand in an Arab country).

Instead of being defensive and mocking us for "not being able to take a joke" (and demonstrating your cultural insensitivity), you might try apologizing for being unintentionally offensive and ask a few questions about the differences between Canadian and American culture. Who knows, you might learn something.