[XX000] ERROR: could not open relation with OID "someoid"

Hi.

When i running query (to find the total size of biggest relation in db):
https://wiki.postgresql.org/wiki/Disk_Usage

SELECT nspname || '.' || relname AS "relation",
pg_size_pretty(pg_total_relation_size(C.oid)) AS "total_size"
FROM pg_class C
LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace)
WHERE nspname NOT IN ('pg_catalog', 'information_schema')
AND C.relkind <> 'i'
AND nspname !~ '^pg_toast'
ORDER BY pg_total_relation_size(C.oid) DESC
LIMIT 20;

I'am often getting the error [XX000] ERROR: could not open relation with
OID "someoid". Every time the OID is different.
Some times the query succeeding and i see the result.
This also happens on the slave servers.

Any ideas?
Thank you.

PS
PostgreSQL 9.1.2 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.4.7
20120313 (Red Hat 4.4.7-11), 64-bit