diff --git a/versions-prod.cfg b/versions-prod.cfg
index f3f2c38bab..716be18f01 100644
--- a/versions-prod.cfg
+++ b/versions-prod.cfg
@@ -20,6 +20,7 @@ Paste = 3.4.3
 PasteDeploy = 2.1.0
 Persistence = 3.0
 Products.BTreeFolder2 = 4.2
+# ZCatalog 6+ no longer supports Zope 4.
 Products.ZCatalog = 5.1
 Record = 3.5
 RestrictedPython = 5.0