diff --git a/etc/zebradb/marc_defs/normarc/biblios/biblio-koha-indexdefs.xml b/etc/zebradb/marc_defs/normarc/biblios/biblio-koha-indexdefs.xml
index d2aef1a72a..5ce689f68d 100644
--- a/etc/zebradb/marc_defs/normarc/biblios/biblio-koha-indexdefs.xml
+++ b/etc/zebradb/marc_defs/normarc/biblios/biblio-koha-indexdefs.xml
@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <kohaidx:index_defs xmlns:kohaidx="http://www.koha-community.org/schemas/index-defs"
                     xmlns="http://www.koha-community.org/schemas/index-defs">
+  <id>marc:datafield[@tag='999']/marc:subfield[@code='c']</id>
   <!--record.abs line 64: xelm /record/leader llength:w:range(data,0,5),rtype:w:range(data,6,1),Bib-level:w:range(data,7,1)-->
   <index_leader offset="0" length="5">
     <target_index>llength:w</target_index>
diff --git a/etc/zebradb/marc_defs/normarc/biblios/biblio-zebra-indexdefs.xsl b/etc/zebradb/marc_defs/normarc/biblios/biblio-zebra-indexdefs.xsl
index 064c8a89e1..56d006b4a1 100644
--- a/etc/zebradb/marc_defs/normarc/biblios/biblio-zebra-indexdefs.xsl
+++ b/etc/zebradb/marc_defs/normarc/biblios/biblio-zebra-indexdefs.xsl
@@ -28,7 +28,7 @@ definition file (probably something like {biblio,authority}-koha-indexdefs.xml)
     </xslo:if>
   </xslo:template>
   <xslo:template match="marc:record">
-    <xslo:variable name="idfield" select="normalize-space()"/>
+    <xslo:variable name="idfield" select="normalize-space(marc:datafield[@tag='999']/marc:subfield[@code='c'])"/>
     <z:record type="update">
       <xslo:attribute name="z:id">
         <xslo:value-of select="$idfield"/>