Is this endgame drawn?

I just finished a correspondence game in which the position at the end appeared drawn, and my opponent and I agreed to a draw: http://en.lichess.org/qg7MTvvH/white#65 . [It was also a good game in that my opponent and I made remarkably few obvious mistakes.]

However, Stockfish evaluates the end position as -2.5 pawns, even though the material is even and it appears that neither side can make progress. Why is this?

33 ... Kc4 wins pretty quickly, no?

I looked at this, and concluded that the king would not be able to break into my position: b4/b5 are defended by my pawns, b3/c3/d3 defended by my king, and c5/d4 defended by my bishop. Am I wrong?

It's not that easy.

White has too many weaknesses. Bishop cannot defend c3, c5 and f4. King cannot move... Black has no weaknesses and a protected passed pawn.

It seems Kc4 + Nd5 wins.