A Montreal family learned an expensive lesson when they recently booked a trip to Cuba but were stopped from boarding their plane because they had signed their children's passports.

The parents weren’t aware that doing so would render the passports invalid according to Passport Canada’s rule that children’s passports must remain unsigned.

When they arrived at the Air Canada check in counter, they realized the mistake.

“As soon as they saw the kids’ passports the lady said, ‘they're signed so they're invalid, I'm going to see my supervisor,’ and she left with the passports,” said Muriel Frenois. “My husband and I were just very anxious and we didn't understand what was going on.”

They had traveled to Cuba with the same passports nine months earlier with another airline.

They say all Air Canada could do was give them a phone number for the passport office.

In order to get an appointment for an express passport, the family had to rebook four return flights to Cuba the following day, and pay more than 500 dollars for the two emergency passports.

In the end they were able to take their vacation, but what was supposed to be a cheap getaway ended up costing them nearly $8,000.

The couple says they realize it was their mistake but are upset with how the situation was handled.

They hope their story will save others from making the same costly mistake. 

Air Canada told CTV that regardless of any previous travel on other airlines they have no choice but to follow the law, and failing to do so would result in fines.