What does it mean if the change of volume of a linear transformation is -1?

Question

Let $P:\Bbb{R}^2\to\Bbb{R}^2$ be projection onto the line with equation $x + 2y = 0$. Find $\det(P)$.

When computing the previous problem, I find that the change of volume is $-1$, however, the determinant is technically $0$ because the linear transformation maps everything to a line that has no volume.