Argument of \caption@ydblarg has an extra }

Question

A misleading error message from the following code:

\documentclass{article}
\usepackage{graphicx}
\usepackage{subfig}

\begin{document}

\begin{figure}[!ht]
\centering%
\subfloat[]{
    \label{subfig1}
    \includegraphics[width=.4\linewidth]{fig1.pdf}%
}%
\qquad%
\subfloat[]{
    \label{subfig2}
    \includegraphics[width=.4\linewidth]{fig2.pdf}
}%
\label{myfig}
\caption{A caption for figures \subref{subfig1} and \subref{subfig2}.}
\end{figure}

\end{document}

gives

! Argument of \caption@ydblarg has an extra }.
<inserted text> 
                \par 
l.19 ...es \subref{subfig1} and \subref{subfig2}.}

?

What have I done wrong? I can't see any extra } anywhere.

You need to protect \subref, i.e. \protect\subref{subfig1} — karlkoeller, Feb 19 '15 at 09:38
Aah. I have no idea what fragile commands and protect are, but now I know where to begin. Thanks very much @karlkoeller! — jozxyqk, Feb 19 '15 at 09:49

score 42 · Accepted Answer · answered Feb 19 '15 at 10:10

The easiest solution in these cases is to add \protect in front of the usual suspects; \subref is one of them because it has a *-variant.

\documentclass{article}
\usepackage{graphicx}
\usepackage{subfig}

\begin{document}

\begin{figure}[!ht]
\centering
\subfloat[]{%
    \label{subfig1}%
    \includegraphics[width=.4\linewidth]{fig1.pdf}%
}%
\qquad
\subfloat[]{%
    \label{subfig2}%
    \includegraphics[width=.4\linewidth]{fig2.pdf}%
}

\caption{A caption for figures \protect\subref{subfig1} and \protect\subref{subfig2}.}
\label{myfig}

\end{figure}

\end{document}

Note where I added/removed % at end of lines and the position of \label{myfig} after \caption.

This might be annoying if you have several appearances of \subref in moving arguments. In this case you can “robustify” the command:

\documentclass{article}
\usepackage{graphicx}
\usepackage{subfig}
\usepackage{etoolbox} % for \robustify

\robustify{\subref}

\begin{document}

\begin{figure}[!ht]
\centering
\subfloat[]{%
    \label{subfig1}%
    \includegraphics[width=.4\linewidth]{fig1.pdf}%
}%
\qquad
\subfloat[]{%
    \label{subfig2}%
    \includegraphics[width=.4\linewidth]{fig2.pdf}%
}

\caption{A caption for figures \subref{subfig1} and \subref{subfig2}.}
\label{myfig}

\end{figure}

\end{document}

Another solution is to avoid subfig for subcaption, which is the path I'd recommend:

\documentclass{article}
\usepackage{graphicx}
\usepackage{subcaption}

\begin{document}

\begin{figure}[!ht]
\centering
\subcaptionbox{\label{subfig1}}{%
    \includegraphics[width=.4\linewidth]{fig1.pdf}%
}%
\qquad
\subcaptionbox{\label{subfig2}}{%
    \includegraphics[width=.4\linewidth]{fig2.pdf}%
}

\caption{A caption for figures \subref{subfig1} and \subref{subfig2}.}
\label{myfig}

\end{figure}

\end{document}

Great answer, thanks! Can you explain why the \qquad line doesn't need a %? — jozxyqk, Feb 19 '15 at 10:25
@jozxyqk A space/end-of-line is ignored after a control sequence (whose name consists of letters). — egreg, Feb 19 '15 at 10:31
The \protect method also solved my problem with an equivalent error using inparaenum inside a caption - had to apply it to \begin, \end and each \item — Bamboo, Jan 27 '16 at 03:22
Another 'suspect' turned out to be the \linebreak. Using \protect\linebreak also helped. — hanna, Jan 25 '20 at 13:11
Using \protect also solves problems caused by hyperlink in captions. — Bromind, Aug 13 '20 at 16:05
I had the same issue with cref from the cleveref package. Adding \protect sorted it. But I was already using the subcaption package, not subfig. Is this to be expected? — Sam OT, Jul 14 '21 at 19:45
@SamOT The command \subref provided by subcaption is robust. Also \cref is; but it would be a different issue anyway. — egreg, Jul 14 '21 at 19:48

Argument of \caption@ydblarg has an extra }

1 Answers1