postgresql: pg_relation_size glitches?

How did pg_relation_size locate something does not exist in the db?

VCDB=# select current_database(), version();

 current_database |                                                                     version

------------------+---------------------------------------------------------------------------------------------------------------

-----------------------------------

 VCDB             | PostgreSQL 9.4.9 (VMware Postgres 9.4.9.1-4536683 release) on x86_64-unknown-linux-gnu, compiled by x86_64-vmk

-linux-gnu-gcc (GCC) 4.8.4, 64-bit

(1 row)

VCDB=# select pg_relation_size(table_name) as Size  FROM information_schema.tables WHERE table_schema NOT IN ('information_schema', 'pg_catalog') ORDER BY size DESC LIMIT 10;

ERROR:  relation "vci_version" does not exist

VCDB=# \d vci_version;

Did not find any relation named "vci_version".

VCDB=#

VCDB=#  select table_name FROM information_schema.tables WHERE table_schema NOT IN ('information_schema', 'pg_catalog');

           table_name

--------------------------------

 cis_kv_providers

 cis_kv_keyvalue

 vpx_alarm_expression

 vpx_alarm_expr_comp

 vpx_alarm

...

SELECT tablename,  pg_size_pretty(pg_relation_size(schemaname || '.' || tablename))

FROM pg_tables WHERE schemaname = 'vc';

           tablename            | pg_size_pretty

--------------------------------+----------------

 cis_kv_providers               | 8192 bytes

 cis_kv_keyvalue                | 8192 bytes

 vpx_alarm_expression           | 112 kB

 vpx_alarm_expr_comp            | 48 kB

 vpx_alarm                      | 104 kB

 vpx_alarm_runtime              | 8192 bytes