diff --git a/lightningd/subd.c b/lightningd/subd.c
index 7dcbb0956..a959eb1d7 100644
--- a/lightningd/subd.c
+++ b/lightningd/subd.c
@@ -836,7 +836,8 @@ void subd_send_msg(struct subd *sd, const u8 *msg_out)
 	u16 type = fromwire_peektype(msg_out);
 	/* FIXME: We should use unique upper bits for each daemon, then
 	 * have generate-wire.py add them, just assert here. */
-	assert(!strstarts(sd->msgname(type), "INVALID"));
+	if (strstarts(sd->msgname(type), "INVALID"))
+		fatal("Sending %s an invalid message %s", sd->name, tal_hex(tmpctx, msg_out));
 	msg_enqueue(sd->outq, msg_out);
 }