The reality star showed off the card she received from Kim, who wished her a happy 41st birthday - even though she was actually turning 42.
'Happy 41st birthday,' Kim, 40, wrote in the card, before the mistake was pointed out to her by Kourtney.
'My bad,' Kim replied.
In true Kim fashion, she presented Kourtney with a card that had her 'ugly' crying face on it with text that read: 'OMG YOU'RE SO OLD!'
And in her thoughtful birthday message she accidentally wished her eldest sister a 'Happy 41st birthday,' despite Kourtney's turning 42 on April 18.
'So Kim thinks I'm 41 so, I guess last year really didn't count,' the girlfriend of Travis Barker said in the video.
Afterwards the SKIMS founder re-posted the video, inserting text underneath that read: 'My bad.'
Despite Kim's age mishap Kourtney was showered on her special day with sweet birthday tributes from her siblings and friends.
'Happy Birthday to my Armenian Queen! The person on this planet I've known the longest! The person that will try any beauty treatment with me LOL about Kourt...There's NO ONE like you!
'You always know what you want in this life and will never conform to what others ideals are and I admire that so much! You always stick up for what's right and have become the best therapist a girl could ask for! I love you too the end of time and beyond! Have the best birthday, you deserve it all!!!,' Kim wrote in her tribute post.
Her mom Kris Jenner gushed that the day she was born at 3:15AM was the 'best day of [her] life.'
And Khloe and Kendall also shared snaps with their sister, Khloe also calling her 'my soulmate!!'
The Blink-182 drummer also aired his feelings about his new lady love in his post as he shared never-before-seen photos of the pair kissing.
'I F**KING LOVE YOU! YOU'RE A BLESSING TO THIS WORLD HAPPY BIRTHDAY @kourtneykardashian.'
