LOS ANGELES, Calif.–More than 6,600 residents of Los Angeles and Orange counties have federal tax refunds waiting to be claimed, but Internal Revenue Service officials said today they don’t know where to send the checks.

“We want to make sure taxpayers get the money owed to them,” said IRS Commissioner Doug Shulman. “If you think you are missing a refund, the sooner you update your address information, the quicker you can get your money.”

The checks owed to Los Angeles County residents total $11.38 million and average about $2,145 per refund recipient, according to IRS spokesman Raphael Tulino. He said the unclaimed checks for Orange County residents total $1.66 million and average $1,229 per refund recipient.

Roughly 15,700 Californians have unclaimed refund checks awaiting them, totaling $23.4 million. Some households are owed more than one check.

For whatever reason, the checks that were sent out earlier this year were returned as undeliverable by the U.S. Postal Service. According to the IRS, 99.68 percent of the checks distributed–via mail or direct deposit–made it to their destinations without a hitch.

Taxpayers can check the status of their refund by visiting IRS.gov and selecting the “Where’s My Refund” option, or they can contact the agency at (800) 829-1954.