diff --git a/doc/man/man5/slapo-pcache.5 b/doc/man/man5/slapo-pcache.5
index 8b1df07db0..f863a3efa1 100644
--- a/doc/man/man5/slapo-pcache.5
+++ b/doc/man/man5/slapo-pcache.5
@@ -80,6 +80,9 @@ or both; in the latter case, any other attribute is redundant and should
 be avoided for clarity.  A set of attributes can contain "1.1" as the only
 attribute; in this case, only the presence of the entries is cached.
 Attributes prefixed by "undef:" need not be present in the schema.
+The "undef" keyword cannot be used with the
+.BR slapd\-mdb(5)
+backend as it requires all schema elements be fully defined.
 
 .TP
 .B pcacheMaxQueries <queries>