The Internal Revenue Service’s Wednesday guidance upended many taxpayers’ plans of paying their 2018 property taxes early to claim a federal deduction before the new tax law takes effect in January.

If you’re thinking of prepaying—or if you have already prepaid—here are a few things to keep in mind:

1. Check your tax-assessment dates

Make sure your local tax office has assessed your property for 2018. Many jurisdictions will bill taxpayers several times throughout the year. If you have received a bill due in 2018, it might make sense to pay it now if you want to claim a federal deduction. For instance, Iowa property taxes are paid semiannually and residents have received a bill due in early 2018, said Nicole Kaeding, an economist at the Tax Foundation. “Those individuals could go ahead and pay by the end of the year and it would be safe to deduct,” she said. In jurisdictions that have assessed properties but not yet sent bills, there may be a gray area. The District of Columbia, which has assessed but not yet billed taxpayers, says they may qualify for the deduction by paying before the end of this year. But it is not yet clear the IRS will agree.

2. Beware the AMT

Make sure that by prepaying your property taxes you wouldn’t become eligible to pay the alternative minimum tax. The AMT is a separate tax system originally designed to prevent people from using legal tax breaks to avoid paying all taxes that largely applies to higher-income households. If prepaying your 2018 property taxes sends your federal tax deductions soaring, you may hit the threshold that would require you to pay the AMT. Similarly, if you will already be paying the AMT, prepaying property taxes may provide little or no benefit.

3. Make sure your state and local tax bill is high enough

Try to determine whether your total state and local tax bill next year will exceed the $10,000 cap that was added to the law. If you have consistently paid less than $10,000 in state and local taxes, you will likely still be able to deduct the full amount you pay in to your state and local governments. Also note that depending on your individual tax situation, you may prefer for 2018 to take the standard deduction, which the law raised to $12,000 for individuals, $18,000 for heads of household and $24,000 for married couples filing jointly.

4. Speak with your mortgage provider

Many homeowners pay their property taxes through an escrow account set up by the mortgage provider. That way they send one monthly payment to the mortgage company, which then takes care of paying the necessary taxes. If that is your situation and you’d like to prepay your property taxes, make sure you speak with your mortgage company first so you don’t end up paying your taxes twice.

5. Talk to a professional

Finally, and most important, talk to your tax adviser. Circumstances vary by household and somebody well acquainted with your situation is probably in the best position to give you relevant advice.