Because the dec should be prorated, so your lease should tell you how much you should pay in dec. Pay it ASAP and the late fee if you missed it in your lease.
If not in your lease, you can argue in the court that the leasing office did not tell you how much you should pay and request leasing office to drop the case after paying.
If not in your lease, you can argue in the court that the leasing office did not tell you how much you should pay and request leasing office to drop the case after paying.