Matrices And Determinants

If $$\mathrm{A}=\left[\begin{array}{cc}\lambda & \mathrm{i} \\ \mathrm{i} & -\lambda\end{array}\right]$$ and \(\mathrm{A}^{-1}\) does not exist, then \(\lambda=\) (where \(\mathrm{i}=\sqrt{-1}\))