pg_relation_size, relation does not exist

Does anyone know why I get an unknown relation error when I query for
relation size?

kls=# select tablename, pg_size_pretty(pg_relation_size(tablename))
from pg_tables where schemaname = 'econ' order by tablename;
ERROR: relation "series_info" does not exist
kls=#

Is there a better way to do this query to avoid this error?

Thanks,
Whit