bucket-engine assert while shutting down tap connections bucket_engine.c:2502

Thread 1 (Thread 19131):
#0 0x00007f934a96ea75 in raise () from /lib/libc.so.6
#1 0x00007f934a9725c0 in abort () from /lib/libc.so.6
#2 0x00007f934a967941 in __assert_fail () from /lib/libc.so.6
#3 0x00007f9349530d63 in bucket_engine_release_cookie (cookie=0x2d58c48)
at bucket_engine.c:2502
#4 0x00007f9346a2daef in TapConnection::releaseReference (
this=0x7f93422b8410, force=187) at tapconnection.cc:35
#5 0x00007f9346a40b18 in TapConnectionReaperCallback::TapConnectionReaperCallback(EventuallyPersistentEngine&, TapConnection*) ()
from /opt/membase/lib/memcached/ep.so
#6 0x00007f9346a3f43d in TapConnMap::shutdownAllTapConnections (
this=0x7f9340023558) at tapconnmap.cc:366
#7 0x00007f9346a0af6f in EventuallyPersistentEngine::destroy (
handle=0x7f9340022fb0, force=true) at ep_engine.cc:1517
#8 EvpDestroy (handle=0x7f9340022fb0, force=true) at ep_engine.cc:97
#9 0x00007f934952ef9f in engine_shutdown_thread (arg=0x7f9340001250)
at bucket_engine.c:1394
#10 0x00007f934acc39ca in start_thread () from /lib/libpthread.so.0
#11 0x00007f934aa216cd in clone () from /lib/libc.so.6
#12 0x0000000000000000 in ?? ()