ERROR: invalid value "????" for "YYYY"

I'm posting this question to
pgsql-general<http://www.postgresql.org/list/pgsql-general/>.
Hopefully someone can share some insights with me.

I have a bunch of tables in the database and in a separate schema. The
tables' names are in this format:

???_???_???_YYYYMMDD

where the last 8 characters is a date.

*When I query either the information_schema.tables or pg_tables extracting
the last 8 characters out and converting it to a date, it works:*

select table_name, to_date(right(table_name, 8), 'YYYYMMDD') blah
from information_schema.tables
where table_schema = '????'
and table_catalog = '????';

*But as soon as I reference it in the where clause, it gives a weird error:*

select table_name, to_date(right(table_name, 8), 'YYYYMMDD') blah
from information_schema.tables
where table_schema = '????'
and table_catalog = '????'
*and to_date(right(table_name, 8), 'YYYYMMDD') is not null;*

*ERROR: invalid value "tati" for "YYYY"*
DETAIL: Value must be an integer.

It seems like some strange values were passed into the to_date function,
but I'm seeing that the rightmost 8 characters of all the table names are
dates. So only date strings are passed to the to_date function.
Absolutely nothing containing the string "tati" is passed to the to_date
function. What is going on? Is that a bug?

Brian