Problem related to begin{case}

This multi-line equation has been making me crazy. Do I have to add math environment for this set of lines?
It gives me errors with braces.
Any help??


\begin{equation}
 \Psi _{ij} =
  \begin{cases}
     \frac{ log \frac{P_{ij}} {P_{0}}} {log \frac{1} {p_{0}} },  \mbox{if } P_{ij} \geqslant p^{*} \\
    -\infty ,  \mbox{otherwise}
  \end{cases}
 \end{equation}

Thanks