Kaitlin Olson and Rob McElhenney expecting second child

(CBS) "It's Always Sunny in Philadelphia" co-stars Kaitlin Olson and Rob McElhenney are adding another boy to their brood.

The couple, already parents to 13-month-old son Axel, told People magazine that they are expecting their second child this spring.

Pictures: Celebrity baby bumps

"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," Olson joked. "We found out I was pregnant during my mom's birthday weekend in Ojai. I was instantly so proud of this baby for already knowing how to make someone else's special day all about him."

Olson, 36, and McElhenney, 34, married in 2008, and Axel was born in September 2010.