[java] Don't know what gentext to create for xref to: "imagedata", ("")

Hi,
I'm generating docbook HTML output using an ant build file. The source XML specifies the following DOCTYPE:

<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
                      "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">

I have been producing the required output correctly for some time, but recently I noticed the following error message when I run the ant build:

 [java] Don't know what gentext to create for xref to: "imagedata", ("")

I have no idea what I have changed to introduce this error. I have noticed that some of my xref links are now generating ??? in the output, where they previously generated the required text.
I use the following markup extensively for xrefs to figures:

<xref linkend="my-figure-id" xrefstyle="template:Figure %n"/>

I'd appreciate any suggestions on what might be causing this error and how to resolve it.

Regards
Christine