diff --git a/doc/src/sgml/cube.sgml b/doc/src/sgml/cube.sgml
index 60bd84bf22b..0a226ca5428 100644
--- a/doc/src/sgml/cube.sgml
+++ b/doc/src/sgml/cube.sgml
@@ -4,7 +4,7 @@
  <title>cube</title>
 
  <indexterm zone="cube">
-  <primary>cube</primary>
+  <primary>cube (extension)</primary>
  </indexterm>
 
  <para>