diff --git a/sources/auth.c b/sources/auth.c
index 4759ef78..1da87441 100644
--- a/sources/auth.c
+++ b/sources/auth.c
@@ -502,7 +502,7 @@ od_auth_backend(od_server_t *server, machine_msg_t *msg)
 	while (1)
 	{
 		msg = od_read(&server->io, UINT32_MAX);
-		if (rc == -1) {
+		if (msg == NULL) {
 			od_error(&instance->logger, "auth", NULL, server,
 			         "read error: %s",
 			         od_io_error(&server->io));
diff --git a/sources/io.h b/sources/io.h
index d0542266..6afe3416 100644
--- a/sources/io.h
+++ b/sources/io.h
@@ -192,6 +192,7 @@ od_read(od_io_t *io, uint32_t time_ms)
 
 	uint32_t size;
 	size = kiwi_read_size((char*)&header, sizeof(header));
+	assert(size > 0);
 
 	machine_msg_t *msg;
 	msg = machine_msg_create(sizeof(header) + size);