diff --git a/sys/kern/uipc_domain.c b/sys/kern/uipc_domain.c
index cabf710ad967..92aac6115f5c 100644
--- a/sys/kern/uipc_domain.c
+++ b/sys/kern/uipc_domain.c
@@ -324,6 +324,15 @@ sockaddr_alloc(sa_family_t af, socklen_t socklen, int flags)
 	struct sockaddr *sa;
 	socklen_t reallen = MAX(socklen, offsetof(struct sockaddr, sa_data[0]));
 
+#ifdef DIAGNOSTIC
+	/*
+	 * sockaddr_checklen passes sa to sockaddr_format which
+	 * requires it to be fully initialized.
+	 *
+	 * XXX This should be factored better.
+	 */
+	flags |= M_ZERO;
+#endif
 	if ((sa = malloc(reallen, M_SOCKADDR, flags)) == NULL)
 		return NULL;