Justin Bieber and Kourtney Kardashian are reportedly back in touch and hooking up yet again.

Although the couple was unable to celebrate Kardashian's recent 37th birthday as she was in Iceland with family, the 22-year-old singer reportedly sent Kardashian a sweet message on her special day.

“Justin didn’t forget about Kourtney’s birthday. He sent her a sweet Happy Birthday message, along with an open invitation to come stay with him for the weekend in any city where he’s performing,” a source told Hollywood Life on Monday. “He said he wants to spoil her to the max and treat her like a queen as a belated birthday gift. He wouldn’t mind doing something extravagant with Kourtney when he’s in New York for his May shows in Brooklyn.”

Last week, a source confirmed to PEOPLE Magazine that Kardashian and Bieber had been "hooking up on and off for a few months now."

"They have hooked up a few times, including recently. It happens in L.A," the source explained. "He has met her kids, but they don't hook up around the kids. It's on nights when she has help and is able to go out... Kourtney is great. Justin thinks it's cool that she is older and hot. She isn't clingy and has her own life. There are no bad consequences for Justin to hook up with her. She is very easy going and just makes it fun for him to be around her.”

Months ago, a source explained details of Bieber's relationship with the Keeping Up With The Kardashians star.

"He has had a huge ego boost from hooking up with Kourt. It's been going on for a few months," a source told PEOPLE Magazine in December. "He is used to being swooned by young girls that have nothing else going on. Kourt is a hot mom with a career. He more thought, 'Wow,' and couldn't believe that she would go for him.' He brags about how hot she is. He is happy while it lasts. Even if she is using him to get back at Scott, he couldn't care less."

For more on Justin Bieber and Kourtney Kardashian, check out the Hollywood Life clip below.