# Error when including letters in Math environment in beamer [duplicate]

I am trying to make a presentation in beamer. However when I include letters in the math environment I get the following error message:

!pdfTeX error: pdflatex (file mathkerncmssi8): Font mathkerncmssi8 at 525 not f
ound
==> Fatal error occurred, no output PDF file produced!


What am I doing wrong?

\documentclass[10pt]{beamer}
\usetheme{Copenhagen}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{graphicx}
\title{Test}

\begin{document}

\begin{frame}
\titlepage
\end{frame}

\begin{frame}
$r_d$
\end{frame}

\end{document}