diff --git a/transport-helper.c b/transport-helper.c
index 4e4754c32b..ba06b70cce 100644
--- a/transport-helper.c
+++ b/transport-helper.c
@@ -973,7 +973,7 @@ static int udt_do_read(struct unidirectional_transfer *t)
  */
 static int udt_do_write(struct unidirectional_transfer *t)
 {
-	size_t bytes;
+	ssize_t bytes;
 
 	if (t->bufuse == 0)
 		return 0;	/* Nothing to write. */