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