XREF to area (callout)

Dear List,
Whenever I try to make a xref to a callout the (FO+HTML) stylesheets
tell me "Don't know what gentext to create for xref to: "area"". 
I'd would expect them to output the corresponding callout icon for the
xref. 
I wonder how to get xrefs to callouts (the callout icons) in the text.

Best wishes
Ludger