BRITNEY WAS LOVELY ON SET!

US Weekly Reports:"She was lovely," Allyson Hannigan told Usmagazine.com at the sitcom's re-launch party in West Hollywood Thursday.

"She’s just a person — so sweet and funny," Hannigan added. "We just talked about our clothes and dogs, and it was surprisingly just normal."

The rest of the cast couldn't stop raving about the actress — who shot scenes Wednesday and Thursday in L.A. — either.

"She’s beautiful," actor Jason Segel told Us. "You know, I was so focused on her hair that I’m overwhelmed with adjectives! She was great and lovely."

Spears' How I Met Your Mother episode airs March 24.