In FoilTeX, with sans serif font, \emph gives serif font

I'm using the foils class. I'd like all the text to be serif, so I added \renewcommand{\familydefault}{\rmdefault} to the preamble. However, when I use \emph{}, it displays italicised SANS serif!

MWE:

\documentclass[landscape]{foils}
\renewcommand{\familydefault}{\rmdefault}
\begin{document}
\foilhead{Test slide}
This word is \emph{emphasized}.
\end{document}

Help greatly appreciated! -Lu