diff --git a/test/unix/test_lwt_process.ml b/test/unix/test_lwt_process.ml
index 7404f336d..8e98bf7e6 100644
--- a/test/unix/test_lwt_process.ml
+++ b/test/unix/test_lwt_process.ml
@@ -35,6 +35,7 @@ let pwrite ~stdin pout expected =
 
 let read_all ic buf ofs len =
   let rec loop ic buf ofs len =
+    assert (len >= 0);
     Lwt_unix.read ic buf ofs len >>= function
     | 0 ->
         Lwt.return ofs