Consider the linear transformation $T: \mathbb{R}^3 \to \mathbb{R}^3$ defined by the matrix $A = \begin{pmatrix} 1 & 2 & 1 \\ 0 & k & 3 \\ 0 & 0 & k-2 \end{pmatrix}$. For which values of $k$ is the transformation not invertible?