Andy Awford fumed at his players for the defensive lapses which saw them fall to defeat to AFC Wimbledon.
Two first-half goals proved the difference at Fratton Park today, as Matt Tubbs and Frankie Sutherland struck.
Both goals came from set-pieces as Sutherland smashed in a 25-yarder before Tubbs fired home a quickly-taken free-kick.
Pompey have now fallen behind in four games on the bounce as they tumble down League Two.
Awford knows the suicidal defending, quite simply, has to stop.
He said: ‘We’ve switched off from two set-pieces and that has cost us.
‘It’s not acceptable. We can’t keep giving ourselves mountains to climb.
‘It’s the past four games we have gone behind now. It’s not on.
‘I’ve caned the players but now I have to pick them up for Luton (on Sunday).’