Why does the following table not compile?

It would be very kind of you if you helped me fix that. Thanks!

\documentclass{beamer}\usepackage{tikz}\usepackage[ruled,linesnumbered]{algorithm2e}\begin{document}\begin{frame}[t]{RPNI - example}    $ S_+ :=\{a,b,aa\} $, $ S_- :=\{\varepsilon, ab, aab\} $\\    \begin{tabular}[l | c]        $ PTA(S_+) $&        a    \end{tabular}\end{frame}\end{document}