programlistingo with fop 0.93

I am using fop 0.93 and  1.72.0 stylesheet and formatting a 
document  with a programlistingco

I have set fop1.extensions
               use.extensions
               callout.extension
               callout.graphics

The output for the callout bug is 
<fo:external-graphic alt=""  ..>

  which fop does not accept because of the alt attribute  
       fo:external-graphic, Invalid property name 'alt

 The fo for the callout list  bug doesn't have  an alt.
  I searched the stylesheet for the code that is writing the alt, bu no luck.
 Is it a fop bug rejecting the alt ?
     Ian