WebHelp bug?

Building one of our books in WebHelp format gives a fatal error:

No template for "/book/chapter/mediaobject/imageobject/imagedata" (or
any of its leaves) exists in the context named "title" in the "en"
localization.
XPath error : Invalid type xmlXPathCompiledEval: 1
objects left on the stack.

A took some investigation to boil down the issue to this:

A construct like this makes WebHelp builds fail:

<mediaobject>
  <imageobject><imagedata .../>...</imageobject>
  <textobject><programlisting>...</programlisting></textobject>
</mediaobject>

The failure doesn't occur when the construct is changed to either of these:

a) Replacing <programlisting> with <phrase>:

<mediaobject>
  <imageobject><imagedata .../>...</imageobject>
  <textobject><phrase>...</phrase></textobject>
</mediaobject>

b) Removing the <imageobject>:

<mediaobject>
  <textobject><programlisting>...</programlisting></textobject>
</mediaobject>