The rush is on to pay next year’s property taxes before the end of this year, meaning there are longer lines than usual this week at the Polk County Treasurer’s Office, KCCI-TV reported.

It’s all thanks to the new federal tax law passed in Congress, which caps property tax deductions at $10,000. So, prepaying now can save you cash later. 

Ben Lacey, CEO of the Polk County Treasurer’s Office, said there are normally 100 Polk County residents who might show up to pay taxes or pay them online during the final week of the year. This week, 900 people had prepaid as of Wednesday.

But the IRS says not so fast, according to the New York Times. In an advisory notice posted to its website on Wednesday, the IRS said paying next year’s installments this year could work, but only under limited circumstances. To qualify for the deduction, property taxes not only need to be paid in 2017, they must also be assessed in 2017 — meaning that homeowners who prepaid their taxes based on estimated assessments, or who tried to pay several years’ worth of taxes at once, will probably be out of luck.

“Those individuals now are not getting the benefits of those prepayments,” Nicole Kaeding, an economist at the Tax Foundation, said of people who paid taxes that had not yet been assessed. “All that you’ve done is provided an interest-free loan to your municipal government.”