feynmf returns "Feynman graph:\Delta", no diagram

\documentclass{article}
\usepackage{feynmf}
\begin{document}\setlength{\unitlength}{1mm}
\begin{fmffile}{samplepics}

\begin{fmfgraph*}(40,30) \fmfpen{thick}
\fmfleft{i1,i2} \fmfright{o1,o2}
\fmf{fermion}{i1,v1,o1} \fmf{fermion}{i2,v2,o2}
\fmf{photon,label=$q$}{v1,v2} \fmfdot{v1,v2}
\end{fmfgraph*}

\end{fmffile}
\end{document}