Mary Lou Steward, a dual American-Canadian citizen living in Blaine, Wash., has been flying in and out of Vancouver on her U.S. passport without a problem for the last six years.

That is, until British Airways refused to let her board a flight to Vancouver from London Heathrow Airport in November.

She was pulled aside after a security agent saw Canada as her place of birth on her U.S. passport.

“U.S. citizens seem to be allowed freely into Canada, whereas, I, with a valid U.S. passport was not allowed to get on the flight to the land of my birth,” said Steward.

The dual citizen was caught up in a new travel rule that came into effect November 10, 2016, that can leave Canadians stranded abroad.

According to the new rule, dual citizens flying with a non-Canadian passport to and through Canada now need a valid Canadian passport to board, if their dual citizenship is with any country other than the U.S.

The rule change should not have affected Steward, who holds both a valid U.S. and Canadian passport, but she appeared to be caught up because of the airline's confusion about the new regulations.

According to the Canadian Immigration and Citizenship website, a valid U.S. passport should be good enough but it’s really up to each airline. The airline can decline boarding if staff are not comfortable with your boarding documents, as they can be fined if they improperly board international passengers who are then denied entry.

“I was told you can't get on this plane unless you have a Canadian passport. I said I do and it's at home. Why is it at home? Well because this has always worked," she said.

Steward was left behind while her ailing husband, also an American dual citizen, was boarded because he had both passports.

When contacted by CTV News, British Airways apologized for the incident, stating: “[It] occurred just after the new Canadian travel regulations were put in place.”

Steward was put up in a hotel and flown to Seattle the next day to get home to Blaine. But next time she’s not taking any chances and says she will carry both passports.

The bottom line is, if you’re a dual citizen from any other country you now need a Canadian passport to get in to the country.

Any non-Canadian citizens coming from a country that doesn't require a visa has to register online for the electronic travel authorization. It costs $7, but again U.S. citizens are exempt.

It's up to you to learn the rules before you fly. More information about the changes and requirements is available here.