diff --git a/include/types/ssl_sock.h b/include/types/ssl_sock.h
index c67085e..716be89 100644
--- a/include/types/ssl_sock.h
+++ b/include/types/ssl_sock.h
@@ -129,7 +129,8 @@
 	struct bind_conf *bind_conf; /* pointer to the bind_conf that uses this ckch_inst */
 	struct ssl_bind_conf *ssl_conf; /* pointer to the ssl_conf which is used by every sni_ctx of this inst */
 	struct ckch_store *ckch_store; /* pointer to the store used to generate this inst */
-	int is_default;      /* This instance is used as the default ctx for this bind_conf */
+	unsigned int is_default:1;      /* This instance is used as the default ctx for this bind_conf */
+	/* space for more flag there */
 	struct list sni_ctx; /* list of sni_ctx using this ckch_inst */
 	struct list by_ckchs; /* chained in ckch_store's list of ckch_inst */
 };