How do I put a label inside a Feynman diagram circle?

I was able to make the label appear to be in the center of the circle by using a phantom (invisible) edge across the circle and setting the label.dist to 0 to place the label in the middle of that edge.

\documentclass{minimal}
\usepackage{grffile}
\usepackage{feynmp-auto}

\begin{document}

    \begin{fmffile}{2ptcorrection1}
    \begin{fmfgraph*}(220,12)
    \fmfpen{thick}
    \fmfleft{i1}
    \fmfright{o1}
    \fmf{plain}{i1,v1}
    \fmf{dashes, left=1, tension=0.3}{v1,v2}
    \fmf{plain, right=1, tension=0.3}{v1,v2}
    \fmf{plain}{v2,o1}

    % This is where the magic happens:
    \fmf{phantom,label.dist=0,label=1}{v1,v2}

    \end{fmfgraph*}
    \end{fmffile}

\end{document}

And this is what it looks like: