A crook is behind bars after picking the worst possible victim to mug in Sydney’s inner suburbs yesterday.

At about 2.40pm, the 25-year-old struck up a conversation with a man on Crown Street at Surry Hills before allegedly pulling a knife on him.

He advanced aggressively towards his victim before realising he’d made a very, very big mistake.

It turns out the man he was trying to mug was an undercover police officer.

The policeman pulled out his badge, with the mugger throwing his knife away and taking off on foot.

The officer gave chase and arrested him a short distance away.

The 25-year-old has been hit with a range of charges and been refused bail.

He faces court today.