One of my colleagues was recently fired. She sent me an email asking if I would be willing to serve as a reference. How can I politely turn her down?

Should I just say "I'm sorry, I don't feel comfortable doing this." ??? Something else?

I'm not comfortable with this because she was doing some shady things while employed (most of which she doesn't know I know about). I think she will probably be pretty surprised that I am not willing to serve as a reference. Agh! This is so awkward! Help!