[tor-relays] Failure from drain-fd

Anybody know what this means? My logs are littered with it. Tor v2.6.9 with Libevent 2.0.21-stable, OpenSSL 1.0.1o and Zlib 1.2.8 on Windows Server 2012 R2. Aug 06 13:26:35.000 [warn] Failure from drain_fd [3892 similar message(s) suppressed in last 7200 seconds] Aug 06 15:26:37.000 [warn] Failure from drain_fd [3526 similar message(s) suppressed in last 7200 seconds] Thanks