diff -up bind-9.3.6-P1/lib/dns/db.c.db_unregister bind-9.3.6-P1/lib/dns/db.c
--- bind-9.3.6-P1/lib/dns/db.c.db_unregister	2004-03-08 03:07:52.000000000 +0100
+++ bind-9.3.6-P1/lib/dns/db.c	2009-04-24 12:58:19.335265774 +0200
@@ -790,4 +790,6 @@ dns_db_unregister(dns_dbimplementation_t
 	isc_mem_put(mctx, imp, sizeof(dns_dbimplementation_t));
 	isc_mem_detach(&mctx);
 	RWUNLOCK(&implock, isc_rwlocktype_write);
+
+	*dbimp = NULL;
 }