Full-time from The DenTwo goals from John Marquis were not enough to prevent Millwall's U21s slipping to their first Professional Development League 2 defeat of the season as they finished up on the wrong end of a 3-2 scoreline against Bristol City at The Den.
The Lions got off to the worst possible start when keeper Denzell Gerrar spilled a low cross at the feet of Wes Burns, who put the visitors ahead from close range in the second minute.
John Marquis almost levelled three minutes later but saw his left foot shot smack the foot of a post before City then hit the woodwork themselves when Sid Nelson deflected a Burns strike onto the far post.
The roles were reversed after the break when Marquis fed Trotter whose drive was saved by City keeper Jojo Wollacott.
Firstly Kevin Krans stabbed home a Lewis Hall cross from close range, before Anthony Edgar netted the third from the penalty spot after being upended by Tom Bender.
Rather than throw in the towel, Millwall rallied and Marquis pulled one back from the spot after he was felled by Louis Carey.
Marquis, who had earlier seen a goalbound header from a Penny free-kick inadvertently blocked by Aiden O'Brien, made it 3-2 with another header from a Penny set-piece.