Bug with lucida fonts in beamer

I’ve got a problem with math formulas in Beamer with Lucida fonts.
Here is a MWE

documentclass{beamer}
usepackage[T1]{fontenc}
usepackage[utf8]{inputenc}
usepackage{lucidabr} 
usepackage[francais]{babel}
%--------------------------------------------------------------------------
title{}
author{}

begin{document}

begin{frame}
$x^{a_n}$
end{frame}

end{document}

It seems that the underscore n in exponent that cause trouble