Do I need a license or certification to groom dogs?

I have been grooming dogs for almost two years now, in the client's home. They know I am self-taught and I do a wonderful job. I've moved to Michigan and would like to continue this business, so I'm wondering if they know I am self-taught and never went to grooming school, is it illegal to groom their dogs in their home? Or do I need a certification? I will be getting a business license though...Thanks!
This grooming takes place in the Client's home, not my own. And yes, I do let them know I am self-taught.
Oh and I will also be getting insurance. I just got a BFA in Photography and I did grooming on the side in the meantime. I want to have a photography business as well as a grooming business, but don't want to spend the time/money in grooming school if I have enough skills right now...