That's a critical piece of information you omitted. It sounds like your client is not aware of the fact that you're a Canadian citizen (?) and did not perform work in the United States.
Sometimes companies will require that you certify that you're a foreigner using a Form W-8BEN. If you're not able to resolve this issue with your client by explaining your status and it's creating issues for you, you should consult with an attorney.
It also sounds like it would be worthwhile for you to consult with an accountant who can educate you about your obligations to your clients and your subcontractors generally.