Alt text not appearing in inlinemediaobject

I have the the following docbook 5 markup:
<para>Click the 
<guibutton><inlinemediaobject><alt>Execute</alt><imageobject><imagedata
 fileref="img/ldms/ldms_button_execute.png" 
depth="1em"/></imageobject></inlinemediaobject> 
button.</guibutton></para>

It is outputting the following HTML:
<p>Click the <span class="guibutton"><span class="inlinemediaobject"><img src="" height="13"></span></span>  button.</p>

I'm not sure why the alt text is not carrying over. Am I using the alt tag incorrectly or is this a bug? I'm using webhelp docbook-xsl-ns-1.78.1.

I did see this page in the XSL Guide:
http://www.sagehill.net/docbookxsl/AltText.html
But it looks to be for Docbook4.

--- 
David Goss, M.A. 
Technical Writer, Laboratory Division 
Frontier Science & Technology Research Foundation 
4033 Maple Road 
Amherst, NY 14226 
(716) 834-0900 x7218