A letter from Alberta Health Services to an Edmonton teen that was addressed to "Treaty Indian" was a data entry error, and not indicative of language used by staff, AHS says.

"This was an inexcusable error and should never have happened," AHS said in a statement on Twitter Thursday afternoon.

A photograph of the letter, sent from the finance department of the provincial health authority earlier this week, was posted on Twitter Wednesday night by Dawn Marie Marchand, an Indigenous artist.

In the tweet, Marchand described the letter, which names the girl further down, as dehumanizing.

Marchand said that the 15-year-old girl's mother, a friend, was very upset by the letter. Marchand asked the mother if she could share it on social media.

The mother does not want to make any public statements, as she feels the post "speaks for itself," Marchand said in an email to CBC News.

"How the mistake was made is irrelevant," Marchand said. "It shows a fairly distinct problem that needs to be identified."

Investigation underway

AHS said its initial investigation shows an error occurred "when historical wording related to treaty status was entered into the wrong field on a patient record, at the time of a hospital visit more than a decade ago.

"Following a more recent hospital visit, our computer system inadvertently copied that incorrect wording, and included it on an invoice which was then sent out to the person," AHS said in statement posted on Twitter.

"​We are confident that this is a one-off incident, and that it is not indicative of language used by AHS staff."

AHS said it will review all wording in billing system databases to ensure the incident isn't repeated.

It has also waived the invoice sent to the teen.

AHS apologized for the "inexcusable error" earlier Thursday

"This in no way reflects the beliefs or values of AHS, and is no way indicative of our relationship with First Nations and Indigenous people," the Twitter statement reads. "All of our employees are expected to treat all people with dignity and respect."

AHS said officials met with the mother and daughter on Wednesday night to apologize and that it is consulting with Indigenous leaders about the incident.

Premier apologizes

Premier Rachel Notley also apologized to the teenage girl Thursday.

"The language is completely inappropriate, it is culturally insensitive, and should never have been used," Notley said in the legislature.

Health Minister Sarah Hoffman assured the legislature the province's investigation would be thorough.

"Our goal is to makes sure that we address what happened in this situation so that it never happens again," Hoffman said.