Kourtney Kardashian was once locked inside a bus when the driver forgot she was on board.

Kourtney Kardashian

Kourtney Kardashian

The 'Keeping Up with the Kardashians' star celebrated her 40th birthday on Thursday (18.04.19), and to ring in the occasion, her mother Kris Jenner shared a "traumatising" story from Kourtney's childhood in which she was left locked inside her school bus after falling asleep, as the driver didn't realise she hadn't left the vehicle.

In a video shared to the YouTube channel for Kourtney's lifestyle website Poosh, Kris said: "One of the craziest stories I remember about Kourtney was when she was a two-year-old, she fell asleep on the school bus, going to school, and the bus driver didn't see her sleeping on the bench because she was so tiny and petite. And the bus driver dropped off all of the kids - Kourtney was still sleeping - he locked up the bus and went out to breakfast, and that was pretty traumatising."

The 63-year-old momager went on to wish her first born a happy birthday, and was seen fighting back tears as she dubbed Kourtney as her "little soulmate".

She added: "Happy birthday, Kourtney. I can't believe you're 40 years old, the time goes by so quickly, and in a blink of an eye, you're 40! I promised you I wouldn't cry on your birthday - at your party - and I'm going to try not to cry now.

"I just want you to know you're the love of my life, you are my little soulmate and my first born, and I love you more than you'll ever imagine. Happy birthday and I wish you [all the] happiness and God Bless. I love you!"

Meanwhile, Kourtney's younger sister Kim was less emotional with her message, as she recalled how "mean" Kourtney was to her when they were growing up.

She said: "I just remember having the saddest memories because Kourtney was so mean to me all the time. She was always finding a way to pick on me or pick on me. A funny little example is when we were in Palm Springs ... she would take, like, grapefruit and oranges and throw them at me and try to get me to fall [off my rollerblades]."