The majority (52 percent) of Obamacare enrollees receiving an advance premium tax credit to purchase Obamacare insurance is facing the prospect of paying back $530 of that tax credit to the IRS, according to a new study from H&R Block. This clawback is reducing the refunds for these taxpayers by 17 percent this filing season.

Under Obamacare, taxpayers earning between 133 and 400 percent of the federal poverty level are eligible to receive a tax credit to help purchase insurance on Obamacare exchanges. This tax credit is calculated using old tax data of the recipients. The credit is advanced ahead of time to the taxpayer’s insurance company. The taxpayer must reconcile at tax time the advance credit received with the actual credit she is eligible for.