Democratic presidential candidate Beto O’Rourke and his wife, Amy, appear to have underpaid their 2013 and 2014 taxes by more than $4,000 combined because of an error in the way they reported their medical expenses, according to tax returns the couple released Monday evening.

They took deductions for those costs without regard to the limit that only allowed that break for medical and dental expenses above 10% of income for people their age. Had they not taken the nearly $16,000 in medical deductions, their taxable income would have been higher. In those years it would likely have been subject to the rates they were paying under the alternative minimum tax.

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“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 Mr. O’Rourke said.

The tax year 2013 was the first that provided different income limits for the medical expense deduction, based on a taxpayer’s age. If a tax preparer didn’t enter the client’s age, the tax software he or she used defaulted to allowing the full deduction instead of no deduction, said Tony Nitti, a CPA at RubinBrown LLP who said he made the same mistake in those years. Mr. Nitti looked at the returns at the request of The Wall Street Journal.

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In those years, Mr. O’Rourke was a member of Congress from Texas.

To read more from The Wall Street Journal, click here.