[Discrete Math] Propositional Calculus Formal Proof

From my textbook:

Let W denote the wff ( A -> B) -> B, which is not a tautology. What's wrong with the following proof?

1. A -> B| Premises
2. A | Premises
3. B | 1, 2, Modus Ponens QED | 1, 3, Conditional Proof Rule

The book states "Line 2 is incorrect, since no part of W requires us to prove something of the form A -> X." Why? Doesn't the sub-expression ( A -> B ) ask exactly this?