pre-existing shared memory block

Not sure this is a bug, but I'm seeing the following message:

pg_ctl: another postmaster may be running; trying to start
postmaster anyway
FATAL:  pre-existing shared memory block (key 9812001, ID 655374)
is still in use
HINT:  If you're sure there are no old server processes still
running, remove the shared memory block with the command
"ipcclean", "ipcrm", or just delete the file "postmaster.pid".
pg_ctl: could not start postmaster
Examine the log output.
pg_ctl: neither postmaster nor postgres running

However, ipcs shows there is no shared memory block in use:

$ ipcs

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch
status
0x00000000 327682     root      644        151552     4
dest
0x00000000 360453     root      644        122880     4
dest
0x00000000 393222     root      644        790528     4
dest
0x00000000 720910     ed        666        42900      1
dest

------ Semaphore Arrays --------
key        semid      owner      perms      nsems

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages