A majority of Obamacare customers have so far had to pay back a portion of their tax credits because they underreported their income, according to an analysis from H&R Block.

The tax assistance giant said Tuesday that so far, 52 percent of customers who purchased health insurance through healthcare.gov or a state exchange would have to pay back an average of $530 in tax credits. This decreases their refund by an average of 17 percent.

H&R Block attributes the error to taxpayers underestimating their household income.

Conversely, the analysis also showed that roughly one-third of Obamacare enrollees overestimated their 2014 income, and will receive an additional premium tax credit of about $365, H&R Block said.

The tax credits are calculated based on how much a taxpayer makes, the state they live in, age and how many children they have.

The news comes as the administration has had trouble implementing the tax portion of the healthcare law. The administration admitted last week that it sent a tax form with a critical error on it to nearly 1 million Obamacare customers, causing delays in filing returns.

This is the first year that taxpayers will have to pay Obamacare’s individual mandate 2014 penalty for not getting insurance, and taxpayers may not know they have to pay the penalty until they file their taxes.

The administration offered such people more time to enroll in Obamacare, from March 15 to April 30, to avoid the 2015 penalty for not having insurance, which is much higher.