Extended knowledge base terms not highlighted in ctx_doc.markup

Oracle 10.2.0.4

"bony fish" is a term in the default knowledge base. I want to extend it so that "salmonid" will match about() queries on the term.

 I created a thesaurus which contained:

bony fish
 NT1 salmonid

I load this thesaurus with ctxload and compile it into the knowledge base  with ctxkbtc

I rebuilt my index.

I do a search on "about(bony fish)". Documents which contain "salmonid" are returned.

I call CTX_DOC.MARKUP with this query and the primary key of a row which contains "salmonid",

the word "salmonid" is not highlighted.

Other records which contain default narrower terms (such as "sturgeon") are highlighted correctly - so it appears I am calling the procedure correctly.