test comment with a backslash: \ (that was it).

This MWE compiles ok with pdflatex, but gives an error with tex4ht

\documentclass[12pt]{article}
\usepackage{amsmath,mathtools}
\usepackage{empheq}

\begin{document}
\begin{empheq}{align*}
x & = y \\
2 x & = 2 y
\end{empheq}

\end{document}

(/usr/local/texlive/2014/texmf-dist/tex/latex/graphics/dvips.def)))
Overfull \hbox (8151.60806pt too wide) detected at line 9
[][][][][][][][][][]
! Missing } inserted.
<inserted text>
}
l.9 \end{empheq}

?

Using texlive 2014 on Linux

>pdflatex --version

pdfTeX 3.14159265-2.6-1.40.15 (TeX Live 2014)
kpathsea version 6.2.0
Copyright 2014 Peter Breitenlohner (eTeX)/Han The Thanh (pdfTeX).
There is NO warranty. Redistribution of this software is
covered by the terms of both the pdfTeX copyright and
the Lesser GNU General Public License.
For more information about these matters, see the file
named COPYING and the pdfTeX source.
Primary author of pdfTeX: Peter Breitenlohner (eTeX)/Han The Thanh (pdfTeX).
Compiled with libpng 1.6.10; using libpng 1.6.10
Compiled with zlib 1.2.8; using zlib 1.2.8
Compiled with xpdf version 3.03

>