Those breathing a sigh of relief that their student loan payments are now in line with their income may want to re-examine 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.

The catch comes with the forgiveness, since you generally have to pay income taxes on any forgiven debt (unless you were in a program for teachers or worked in a public service job, in which case the taxes go away). For many people, especially those who finished graduate or professional school with six figures of debt, the tax bill could be well into the five figures. And when it comes, you are supposed to pay in full, immediately.

Figuring out just how many people will be in this situation — and just how high the tax bill could be — is a tough task, and not many experts have tried it.