Oracle Database Discussions

When I execute the query, I do not get the error:

SELECT claim_header_sid, '2' AS "table_area"
FROM claim_header
WHERE tcn = 002006073000410000

But, if I execute:

SELECT claim_header_sid, '2' AS "table_area"
FROM ad_claim_header
WHERE tcn = 002006073000410000

I get the error, unless if I put single quotes for the tcn value.
What could be the problem? The datatype is varchar2(21 bytes).

Thanks.