pgf matrix in a frame in beamer

[shreyasbabu]

Hi all
\begin{frame}[fragile]
  \begin{tikzpicture}[font=\scriptsize]
    \matrix [matrix of nodes,nodes={text width=20mm}]
      {
         a & b \\
      }
   \end{tikzpicture}
\end{frame}

I tried to use this pgf matrix in a beamer frame.
But I get the following error when I specify the `text width' option.
Please help.

! Use of \@iminipage doesn't match its definition.