Some U.S. taxpayers will get a surprise financial boost by the end of January.

The Internal Revenue System announced it will send a new wave of stimulus payments worth $2.4 billion to Americans who missed previous payments. The agency said it discovered about one million eligible taxpayers who never claimed the refund credit on their 2021 tax return.

Single payments are up to $1,400.

IRS Commissioner Danny Werfel said in a news release that the payments would be automatically direct deposited or sent by check in most cases by the end of January. Eligible taxpayers do not need to take any action to receive the checks. They will receive a letter informing them of the incoming stimulus payment.

“The IRS continues to work hard to make improvements and help taxpayers,” Werfel said in the release. “To minimize hassle and make this money available to eligible taxpayers, we are making these payments automatically so that these individuals do not have to go through the extensive process of filing an amended tax return to receive them.”

Recipients include those who filed a 2021 tax return but either left the Refund Credit field blank or entered zero when they were actually eligible for the payment. To check eligibility, taxpayers can check their 2021 tax returns to see if they left the Recover Rebate Credit field blank or entered a zero.

Taxpayers who did not file a 2021 tax return could still be eligible for the stimulus check if they file their 2021 tax return and claim the refund credit by April 15, 2025, the IRS said.

Stimulus payments of up to $1,400 have been sent to Americans as part of a $1.9 trillion COVID-19 relief bill. To receive a check, Americans had to earn less than $75,000 a year or less than $150,000 as a household.

The checks were one of three rounds of stimulus payments during the pandemic, totaling $814 billion. In March 2020, eligible individuals under the CARES Act received up to $1,200 per income taxpayer and $500 per child. In December 2020, eligible individuals received up to $600 per income taxpayer and $600 per child under the Consolidated Appropriations Act. In March 2021, eligible individuals received up to $1,400 per income taxpayer and $1,400 per child under the American Rescue Plan Act.

Most taxpayers eligible for the stimulus payments have already received a check, the agency said.