See https://ci.appveyor.com/project/aantron/lwt/builds/23560442#L309. The test was temporarily patched here: https://github.com/ocsigen/lwt/blob/5446d81fa1000317818346d230eb5ece4e78c962/test/unix/test_lwt_unix.cppo.ml#L86 This should be undone.