pdfx and “undefined control sequence” caused by Subject meta-tag

Question

According to the pdfx documentation it should be possible to add a Subject field to the XMP metadata file. Addition of this field however seems to cause an unexpected error during compilation of the LaTeX source into a PDF/A-1b file (pdfLaTex).

Try to compile the following source:

\documentclass[a4paper, 12pt]{article}
\usepackage[a-1b]{pdfx}
\begin{document}
Hello, World!
\end{document}

using this metadata:

\Author{Me}
\Title{Hello, World!}
\Subject{Hello, World! example.}
\Org{Me}
\Keywords{Hello\sep World}

The source compiles without errors after omission of the Subject tag.

Thanks.

Answer 1

Edit: I found a solution with two patches.

First patch (pdfx.sty):

52c52,53
<  \let\xmpSubject\xmpKeywords
---
> \def\Subject#1{\gdef\xmpSubject{#1}}
>  \let\xmpSubject\@empty

Second patch (pdfa-1b.xmp):

34a35,37
>    <dc:description>
>      <rdf:Alt><rdf:li xml:lang="x-default">\xmpSubject</rdf:li></rdf:Alt>
>    </dc:description>

With this two patches, all is fine (for Subject).

Answer 2

This look like a bug in pdfx: \Subject is documented, but I can find no implementation. Please report it to the authors.