Democratic presidential hopeful Beto O’Rourke and his wife appear to have underpaid their taxes in 2013 and 2014 because of an accounting error with their medical expenses, a report said Monday.

O’Rourke released 10 years of his tax returns on Monday night, which showed the couple likely underpaid their taxes those years by about $4,000, The Wall Street Journal reported.

O’Rourke and his wife Amy took medical deductions those years that were not supposed to be available to a person his age, according to the report.

2013 was the first year the medical deduction was based on a filer’s age, and if O’Rourke did not enter his age into the tax software he used, it would have automatically given him the deduction, according to the report.

If the couple did not take the medical deductions, their combined incomes for those years would have been higher, and would have likely been taxed at the same rate the couple was paying under the alternative minimum tax, The Journal reported.

“After becoming aware of this error, the accounting firm that prepared the filings was immediately informed and will file an amendment as appropriate,” an aide to O’Rourke told the Journal in a statement.