AceShowbiz - John Stamos was forced to sign a non-disclosure agreement to avoid spilling the identity of the celebrity behind the Squiggly Monster costume on U.S. reality show "The Masked Singer".

The contestant was eliminated from the singing competition's season four on Wednesday (04Nov20), when it was revealed comedian Bob Saget was the man in the wacky disguise.

All participants are required to keep the news of their involvement in the show under wraps, but Saget couldn't help but share the secret with his best friend and "Full House" co-star Stamos - so the actor had to sign a legally-binding contract with show producers to prevent the news leaking before his big reveal.

"John Stamos had to sign an NDA, as did my wife and one of my daughters...," Saget told TheWrap of the loved ones who were let in on the singing secret.

"He was like, 'How's it going? Are you having fun?' Because he's like a brother to me and cares."

However, Saget didn't tell his TV daughter, actress Candace Cameron Bure - even when she quizzed him about her hunch.

"After the show last week, I was sent a video by Candace and Marilu Henner in the background, and she's screaming, 'Are you Squiggly Monster?! That's you! I know your voice! I know your movements! You're Squiggly Monster.' "

"And I answered her, I said, 'I don't know what you're talking about. I'm very flattered and love you.' But she knew. She was like, freaking out, because she loves the show because it's a show you can watch with your whole family."

The popular series features celebrities belting out songs while dressed up in elaborate disguises, with judges Ken Jeong, Jenny McCarthy, Nicole Scherzinger, and Robin Thicke rating their performances as they try to guess who is behind the mask.

The least favourite is booted from the show at the end of each episode, when they take off their costumes to reveal their identities.

Other stars kicked off the current season of "The Masked Singer" so far include rapper Busta Rhymes and actors Mickey Rourke and Brian Austin Green.