Those breathing a sigh of relief that their student loan payments are now in line with their income may want to reexamine the rules that set the payment in the first place. There could be a tax time bomb looming, slowly ticking away. And defusing it is not a big part of the policy discussion in Washington at the moment.

This potential tax bill is a byproduct of federal efforts, including the newly expanded income-based repayment program, that allow you to limit the monthly payments on most federal loans to what you can afford to pay. There’s a formula that uses your income to determine your payment. Then, the federal government forgives any remaining balance, usually after 10 to 25 years.