Writing It Up · Medium problem

"Proof" that $1 = 0$: (1) Let $x = 1$. (2) Then $x^2 = x$. (3) So $x^2 - 1 = x - 1$. (4) So $(x + 1)(x - 1) = x - 1$. (5) So $x + 1 = 1$. (6) So $x = 0$. Which step is invalid?