Which error is this?

[Angrboda]

The logic is valid -- the contrapositive of a true statement is also a true statement -- but the syllogism is unsound because not all the premises are true.

e.g.

1) If God does not exist, moral knowledge does not exist;
2) The contrapositive is also true, therefore, if moral knowledge exists, then God exists;
3) Moral knowledge exists:
C) Therefore God exists.

That's valid, but because #1 isn't true, the conclusion isn't valid.