Kendall, 17, decided to sneak into Kylie's room to scare-congratulate her and later uploaded a clip of the incident to her Keek account.
The clip begins with Kendall slowly walking toward her sister's room and abruptly opening the door as she yells "Happy Birthday!" startling both Kiley and a friend.
"Jesus Christ!" Kiley says. "I thought it was like an earthquake crumbling."