diff --git a/test-server/test-server-http.c b/test-server/test-server-http.c
index 4e05b521..c9d635c8 100644
--- a/test-server/test-server-http.c
+++ b/test-server/test-server-http.c
@@ -354,7 +354,8 @@ int callback_http(struct lws_context *context, struct lws *wsi,
 			if (m != n)
 				/* partial write, adjust */
 				if (lws_plat_file_seek_cur(lws_get_fops(context),
-							   pss->fd, m - n) < 0)
+							   pss->fd, m - n) ==
+							     (unsigned long)-1)
 					goto bail;
 
 			if (m) /* while still active, extend timeout */