if the number of objects is divisible by 3 the generated object_index.sgml is wrong:

...
<row>
<entry><link linkend="GInitiallyUnowned">GInitiallyUnowned</link></entry>
<entry><link linkend="GInterface">GInterface</link></entry>
<entry><link linkend="GObject">GObject</link></entry>
</row>
</row>                                       ← wrong, additional closing tag
</tbody></tgroup></informaltable>

and these are the errors:

../xml/object_index.sgml:103: parser error : Opening and ending tag mismatch: tbody line 12 and row
</row>
      ^
../xml/object_index.sgml:104: parser error : Opening and ending tag mismatch: tgroup line 8 and tbody
</tbody></tgroup></informaltable>
        ^
../xml/object_index.sgml:104: parser error : Opening and ending tag mismatch: informaltable line 7 and tgroup
</tbody></tgroup></informaltable>
                 ^
../xml/object_index.sgml:104: parser error : Extra content at the end of the document
</tbody></tgroup></informaltable>
                 ^