Amsmath problem

I wrote this mini-example:

\documentclass[12pt,fleqn,A4,danish]{article}
\usepackage{amsmath}
\usepackage{graphicx}
\usepackage[text={14cm,20cm},centering]{geometry}
\begin{document}
\title{Amsmath problem}
\author{}
\date{}
\maketitle
Bla bla $\mathrm{AuCl_4^−}$ and $\mathrm{TOA^+}$

\end{document}

and get this result:

...
! Missing { inserted.
<to be read again>
                   \begingroup
l.10 bla bla $\mathrm{AuCl_4^−}
                                 $ and $\mathrm{TOA^+}$
?

On a suggestion of a friend I tried  ${\mathrm{AuCl_4^−}}$

with the same result. I also tried his suggestion
to use mhchem, i.e.

\ce{[AuCl4]^-}

and this didn't work either (yes, I did add
\usepackage{mhchem} )

This is supposed to work, and does on my
friend's machine. Could this be a defective
texlive, or am I really doing something
wrong?

--
db