Against a knight pawn on the fourth, the defender must be able to meet Kb5 with Kb7 and Kc5 with Kc7; only the square b8 keeps both answers available.
In Gligorić–Fischer (Candidates 1959) Fischer, as Black, reached this position and found the one move that holds. White has a knight pawn on b4 and a king on c4; Black's king is on c8.