Kourtney Kardashian has had lots of reasons - his partying ways, his laziness, his rude remarks, his temper, etc. -- for postponing her nuptials to longtime boyfriend  Scott Disick, but now she can add "neglectful" to that list.

On Aug. 10, Disick left their 2-year-old son Mason in his Mercedes SUV with the windows up - on a day when temperatures reached as high as 107 degrees in Calabasas, Calif. - while he was browsing magazine at a newsstand, an observed told the National Enquirer.

"He was out of the car for at least five minutes and should have never left his son alone!" the source told the Enquirer, according to RadarOnline. "The car was parked in the hot sun with the boy alone in the back. The car was not running, so the air conditioner wasn't on, and the windows were rolled up."

Kardashian was angry with her partner about his carelessness, and commented that someone should go along with Disick to supervise visits so nothing like this happens again.

"All she could think about is that Mason could have died. She read Scott the riot act, and so did her sisters Kim and Khloe," the insider told the Enquirer. "They don't want Scott taking the kids anywhere now unless someone is with him."

Disick is now the father of six-week-old Penelope Scotland.

"Nothing could prepare me for how hard I fell in love with her," Kardashian told Us Weekly. "With Mason, we have such a love affair that I never knew existed. I didn't expect it to happen again. It's such a magical feeling."

Kourtney added that Mason, 2, is just as in love with his new sister, and he's already developed a strong bond with Penelope.

"He's adjusting to the whole thing so well," she said. "He calls her 'sister.' He gets mad if anyone calls her anything else. People will be like, 'What's your sister's name?' And he'll say, 'Sister!'"

Tags: Kourtney Karashian scott disick