Fix a warning.
Fix a warning.

mfp/lwt,c-cube/lwt,ocsigen/lwt,rgrinberg/lwt,rneswold/lwt,mfp/lwt,rgrinberg/lwt,rgrinberg/lwt,rneswold/lwt,c-cube/lwt,ocsigen/lwt,rneswold/lwt,ocsigen/lwt,rgrinberg/lwt,c-cube/lwt