An indexterm directly below a varlistentry causes a warning like this from FOP:
Page 2: Unresolved ID reference "idp9317436" found.
To reproduce, process the attached example document
xsltproc -o test.fo http://docbook.sourceforge.net/release/xsl/current/fo/docbook.xsl test.xml
(or also with
--param variablelist.as.blocks 1) and then with
fop -fo test.fo -pdf test.pdf
Also attached is a proposed fix.