Using siunitx in captions?

Question

I am using pdflatex(texlive 2011) to compile a document. Unfortunately it does not compile.

! Undefined control sequence.
\metre ->\ERROR 

l.69 ...feld  \num{6}{\metre} mal \num{6}{\metre}}

siunitx works for units in normal text, but using it in captions fails. Is this a bug or misconfiguration. Under texlive 2009 it compiles flawlessly.

Here is my figure command

\documentclass{article}
\usepackage{siunitx}
\begin{document}
\begin{figure}
  \caption{\num{6}{\metre}}
\end{figure}
\end{document}

Answer 1

Things will work correctly if you do not abuse \num :-)

\documentclass{article}
\usepackage{siunitx}
\begin{document}
\listoffigures
\begin{figure}
  Some filler
  \caption{A caption \SI{6}{\metre}}
\end{figure}
\end{document}

Reason: the \num macro command takes one argument: a number. You therefore effectively have a free-standing unit after the \num, and these are not therefore correctly processed by siunitx.

---

To explain why this worked in TL2009, that had version 1 of siunitx. The older code created 'free-standing' units as standard, which was changed for version 2. 'Free-standing' units have various issues, including namespace issues: this change was quite deliberate. Of course, this means that something like

\documentclass{article}
\usepackage{siunitx}
\sisetup{free-standing-units}
\begin{document}
\begin{figure}
  \caption{\num{6}{\metre}}
\end{figure}
\end{document}

will work, but as it's not the correct use of \num I really don't encourage it!