Nobody was marked in the penalty box, Dembele, the goal scorer, Kagawa all were unmarked because it was a free kick and everyone and their mum expected Johnstone to hold it , or at worst push it into touch.. not push it right back past his defence to the attackers behind them. It was as choolboy error for the keeper that wrongfooted the entire defence.
I'm still waiting for a logical explanation on how Valencia (the only defender who got in a block in that sequence) was as culpable as Johnstone, or even Bailly who was ballwatching the whole time while the man closest to him finally put it in the net.