(NewsNation) — More than 500 living patients received letters indicating they were deceased and included information on how their next of kin could resolve their estates following a computer system error.

MaineHealth Medical System, which includes hospitals and clinics in Maine and New Hampshire, said the error occurred on Oct. 20 when its software for getting estate vendor letters malfunctioned, according to multiple reports.

Notices were sent to 531 living patients, though the letters are typically intended for use in cases where patients have died.

The health care system said the patients affected were not listed as deceased in their medical records, and the error didn’t affect their care or status in the system.

A woman from Sanford, who asked to remain anonymous, told WGME-TV that the incident was “pretty upsetting” to receive the letter.

“Why would they say I was dead? So, it was really shocking and upsetting,” she told the outlet.

MaineHealth has since apologized to all affected patients.

“MaineHealth sincerely regrets this error and has sent apology letters to all patients who have been affected,” a spokesperson told ABC News. “At no time were these patients listed as deceased in their medical records and the issue has been fully resolved.”