diff --git a/servers/slapd/attr.c b/servers/slapd/attr.c
index ee0f46daab..5e0b3e6e99 100644
--- a/servers/slapd/attr.c
+++ b/servers/slapd/attr.c
@@ -192,6 +192,13 @@ attr_merge(
 #ifdef LDAP_COMP_MATCH
 		(*a)->a_comp_data = NULL;
 #endif
+	} else {
+		/*
+		 * FIXME: if the attribute already exists, the presence
+		 * of nvals and the value of (*a)->a_nvals must be consistent
+		 */
+		assert( ( nvals == NULL && (*a)->a_nvals == (*a)->a_vals )
+				|| ( nvals != NULL && (*a)->a_nvals != (*a)->a_vals ) );
 	}
 
 	rc = value_add( &(*a)->a_vals, vals );