As the first round of child tax credits land in people’s checking and savings accounts Thursday, many people will embrace the additional income. But for some taxpayers, the advances will create a tremendous headache during the next tax season.

The payments, which will go out on the 15th day of every month from July to December, are based on 2020 returns – so people who face changes between then and now could find some unexpected penalties.

For instance, right now, single filers whose modified adjusted gross income is less than $75,000 per year and couples filing jointly earning $150,000 or less per year qualify for the full child tax credit for each of their dependents. If your family’s income increases in 2021, though, you could be forced to pay back some or all of the prepayments on your 2021 taxes.

If your income stays the same or goes down, you could still be affected — especially if you count on a refund each April. Since the credit is a dollar-for-dollar advance on future taxes, it will likely result in either smaller checks or unexpected tax bills.

“Because these credits are paid in advance, every dollar you receive will reduce the amount of Child Tax Credit you will claim on your 2021 tax return,” warns the IRS. “This means that by accepting advance child tax credit payments, the amount of your refund may be reduced or the amount of tax you owe may increase.”

That could be an even bigger headache depending on which tax year the IRS based the prepayment eligibility on. In most cases, it was 2020, but in some cases, the agency used 2019 returns.

For divorced parents, it could be even more complicated. Families that have just one child often alternate which parent claims the child on their taxes from year to year. Assuming the credit is split, it could leave one parent in the lurch come tax season, when they’re received a child tax credit, but aren’t claiming the deduction that year.

If you suspect you could be negatively impacted by the payments, you can stop or change them. Head to the IRS’s child-tax-credit update portal and follow the instructions. Note that there are deadlines to unenroll. It’s too late to opt out of the July payment if you haven’t already. The next deadline to opt out is coming up on August 2.

A couple things to note: The system is under heavy demand today, so expect possible delays when opting out. Also, both spouses will have to do so separately. (Another option is to adjust your paycheck withholding or estimated quarterly tax filings.)

This story was originally featured on Fortune.com

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