Married It's Always Sunny In Philadelphia co-stars Kaitlin Olson and Rob McElhenney are expecting their second child.

The actress gave birth to the couple's son, Axel Lee, last year and now the star is pregnant with another boy.

She jokes to People.com, "We're having a boy! We're so happy that Axel gets to have a brother so close in age. And that I get to add another set of balls to my very, very male world. We found out I was pregnant during my mom's birthday weekend in Ojai (California)."

"I was instantly so proud of this baby for already knowing how to make someone else's special day all about him."

The pair wed in 2008.

The new child will be the latest addition to the It's Always Sunny in Philadelphia stars' onset family - Glenn Howerton and David Hornsby both had kids last month, while Charlie Day, who is married to his onscreen love interest Mary Elizabeth Ellis, will become a first-time father in December.