If you think you’re missing a $500 payment for your dependents, the window to claim the money is quickly closing. You have until Sept. 30 to submit information on your dependents and correct any errors from the first time around with the IRS’ Non-Filers tool.

The $500 sum should’ve been part of the stimulus checks the IRS started sending out last spring. If you’re unsure of the money you’re owed, try CNET’s stimulus check calculator and your AGI for an estimate. If the size of the check you received doesn’t seem as big as expected, you may need to request a catch-up payment.

If you still haven’t received your stimulus check and you think it’s lost or missing, try this IRS phone number to file a report.

Second stimulus check negotiations are poised to resume at long last, with House Democrats now working on a new coronavirus relief bill reportedly worth $2.4 trillion. Below, we detail everything we know about how to claim a $500 stimulus catchup payment. We regularly update this story.

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I think I’m missing my $500 dependent stimulus check. What should I do?

To file for your missing $500 stimulus money, visit the Free File Fillable Forms site, select Get Started and create an account if you haven’t already. Then follow the posted instructions for filling out the form. You’ll need to provide your full name, mailing and email address, date of birth, Social Security number, bank account number (if you have one), driver’s license or state ID (if you have one), each qualifying child’s Social Security number and the child’s relationship to you.

When you’re finished, you’ll receive a confirmation email. Your information will then be sent to the IRS so they can work on getting a check sent out to you, assuming you’re eligible.

When will my $500 catch-up payment arrive?

The IRS says it will begin issuing checks in October for those who meet the application deadline; you shouldn’t need to do anything else. However, if you’d like more information, you’ll be able to track the status of your IRS payment by visiting the IRS Get My Payment webpage. From there, you’ll need to provide your Social Security number, date of birth, home address and ZIP code.

What if I miss the Sept. 30 cutoff date?

If you miss the September deadline, you’ll have to wait until 2021 to claim your stimulus payment on behalf of eligible dependents. However, the IRS indicates that you shouldn’t use the tool yet if you still plan on filing a 2019 tax return (for instance, if you filed for an Oct. 15 extension), as it will slow down the process.

How do I know if I’m eligible for the first stimulus payment?

To be eligible, the filer must be a US citizen, a permanent resident or qualifying “resident alien.” They must also have a valid Social Security number, can’t be claimed as a dependent of another taxpayer and must have an adjusted gross income — that’s your AGI — under $146,500 (heads of household) or $198,000 (married couples filing jointly).

The CARES Act stipulates a $500 allowance per child dependent in addition to the $1,200 cap for single filers and up to $2,400 for couples filing jointly. (We’ve calculated how much money you might be able to get if rules about dependents change for a second stimulus check.)

Here’s how the IRS defines a child dependent — there may be specific details you’ll want to explore if your child dependent is adopted, disabled (of any age) or a citizen of another country.

For more information about stimulus payments, here’s how fast the IRS could send out a second stimulus payment and the stimulus check requirements. Also, will the stimulus package be split into smaller bills? Here’s what we know.