LaTeX Beamer - Make text in math mode also serif

I am using a sans-serif font for my beamer slides but the regular serif math font for my equations. However, if I use text{} in math mode, the sans-serif font is used. I would like to enforce that the serif font is used when using text{}.

Here is a MWE

documentclass{beamer}
usefonttheme[onlymath]{serif}
begin{document}
begin{frame}
I would like to make $f_text{LO}$ look like $f_mathrm{LO}$.
end{frame}
end{document}

Update: I found out that I should not redefine text{} but use textrm{} instead to use the serif font in math mode. By doing that, I can still add regular text using text{}.