# Does longtable not support the m column type?

I have the following problem: I would like to use the m column type in a longtable. But if I typeset

\documentclass{minimal}
\usepackage{longtable}
\begin{document}
\begin{longtable}{p{5cm}m{5cm}}
skdlbf & asljkdh\\
\end{longtable}
\end{document}


I get an error about an Illegal character in array arg in the line of \begin{longtable}{..}. If I change the m to a p, all runs well.

So why does longtable not support the m (and b) column type? Am I missing a package?

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\usepackage{array}