Kourtney Kardashian has no doubt Scott Disick is their son's biological father.

The 34-year-old reality TV star is facing a lawsuit from model Michael Girgenti, who claims he had unprotected sex with her after they met on a photo-shoot in August 2008 and filed paperwork at Los Angeles County Superior Court this week in a bid to get joint custody of her three-year-old child, Mason.

But the 'Keeping Up with the Kardashians' star insists there is no way Michael is Mason's real dad, as they never had sex.

A source close to Kourtney told gossip website RadarOnline.com: ''She did not have sex with that model. This whole thing is so ridiculous. She thinks it is stupid and it really isn't true.''

The insider added: ''Kourtney knows who the father of her son is and it isn't this guy.''

Michael wants the judge to order DNA tests for Kourtney, Mason and her long-term boyfriend, Scott, 30.

Scott also has a 13-month-old daughter, Penelope, with Kourtney.

A hearing has been set for next month.

The couple has yet to comment on the allegations but were spotted looking miserable on a dinner date at Mr. Chow in Malibu on Friday night (09.08.13).