By People Staff
Updated September 12, 2008 12:00 AM
Credit: Vera Anderson/WireImage; Jason Kempin/WireImage

Jennifer Lopez was supposed to be the surprise guest judge for Project Runway‘s season finale. But an injury auf’d her from the Bravo reality show.

At the Friday morning taping of Runway’s Fashion Week show for its finalists, Heidi Klum announced to the crowd that the singer-actress had to pull out “due to a foot injury” and wouldn’t be able to help decide this season’s winner.

“She hurt her leg/foot, apparently,” a source tells PEOPLE.

Lopez’s appearance on the finale was never publicly confirmed, because producers like to keep the guest judge as much of a surprise as possible, added the source. Instead, Tim Gunn, Runway‘s resident fashion mentor, will be stepping into her shoes. And although he has close relationships with the contestants because he advises each of them on their work, he promised to be “unbiased” as a judge.

“It just seemed totally fitting because he’s never been a judge,” the source says, noting that Gunn “was extremely nervous” to switch roles on the show.

If anything, news of Gunn stepping in as a replacement was greeted warmly by the contestants. “People were crying because they were so moved by Tim being a judge,” says the source.

Lopez has been training to compete in the 2008 Nautica Malibu Triathlon this weekend, a benefit for Childrens Hospital Los Angeles. –By Jessica Gold Haralson and Brian Orloff

Vera Anderson/WireImage; Jason Kempin/WireImage