An Antiques Roadshow hopeful was left gutted after being told his Banksy was worthless - so long as he didn't have a certificate proving it's legit.
The artwork showed a rat holding onto a power drill, and was found at Brighton's Saltdean Lido.
Part of the previously unseen Best of the Summer show, expert Rupert Maas was told by the man he'd nicked the piece from a wall.
"I used to live in Brighton in the late 90s, early 2000s," he explained, "and I was walking along the Brighton seafront when I saw it on the Lido.
"It looked loose, I went over, pulled it off basically."
He added: "I know what it is, I know what year it was, it was around 2004, I'm basically just trying to get a valuation of it."
Rupert pointed out that Banksy - who's identity has long been a mystery in the art world - "manages his brand very, very carefully."
This means that you can apply for a certificate of authenticity, which means a team has looked it over and given it the thumbs-up.
Rupert adds that the team will only give a certificate if "the work has not been removed from the public domain for which it was painted, and into the private."
The man told Rupert that he'd tried to get a certificate, but they "couldn't claim if it was an original Banksy."
"I know it's real," the man told, "because Brighton was hit quite a bit by Banksy when he was down there around that time."
Rupert told him: "I think the message is, if you do see a piece of graffiti art out there, leave it for the public. I'm not lecturing you, I'm just saying, but without that certificate, it's just very difficult to sell.
"With it, it might be worth £20,000. Without it, you're nowhere. I'm sorry."
Back in 2014, a Banksy painted on a door in Bristol was brought in, and eventually sold for a cool £400,000.
*Antiques Roadshow returns on Sunday at 7pm on BBC One.
