Finance Charge on a ZERO balance card.

I got an email stating I owe a finance charge. I logged into my account and all what it shows is the amount and finance charge in the description. How could a bank charge a finance charge on a credit card that has a ZERO balance? I haven’t used this card since March and it has been paid in full with a ZERO balance. I even verified it with them before leaving the bank and they said I didn’t own anything. How could they charge me a finance charge on nothing? I plan on calling them tomorrow. Has this happened to anyone. Why do they do that?