Before you call Child Protective Services, know that we're pretty sure the babysitting sesh was pretty chill. Kourtney actually posted a picture of Mase on Bieber's back in the pool looking like they're having a blast.
In fact, Kris reposted Kourtney's Instagram, writing, "#Mason & @JustinBieber family pool day!!! #bestbabysitterintheworld #Repost @justinbieber jQuery191031038557663608723_1427701606735?Mason is cool as a fan."
Badass Bieber has been spending lots of time with the Kardashian-Jenner clan lately. He recently appeared with Kendall Jenner in Vogue's new "Brat Pack" spread.
Surprised Kourtney lets her kids hang with the Biebs? Or do you think it's kind of cute?