indexterm inside part/title and fo output

Hi,
I have a document where an indexterm appears inside a book/part/title.
Apparently, the part/title element does not get its id during the
index generation and  in the fo generated index the relevant
<fo:page-number-citation ref-id="d0e10266"/> points to an undefined
id.

If I move the indexterm inside a chapter/title everything works as expected.

Any idea on the template controlling the index id generation on part/title ?

Thanks in advance,

__peppo