I have a mortgage and credit card loan, and had a car loan all through large local banks. They were written differently. The mortgage and credit card required that a minimum payment be made each month regardless of overpayment. The car loan was different in that when I overpaid it, the next payment was not due until months later. At the end of that loan the next payment was not due until a year and a half later. So, these loans were obviously written differently.
You should contact the lawyer that wrote the contract. But the question is, if your lawyer neglected to include this specific language in the contract, which of the two above scenarios does the contract default to?