ORA-01847 when to_char is used in where clause

I have the following problem:

select distinct to_char(wpa_timestamp, 'YYYY') from view_waiting_time_book
where to_char(sysdate, 'YYYY') = '2010';

gives me 13 rows back with distinct years, all ok so far. But the following:

select distinct to_char(wpa_timestamp, 'YYYY') from view_waiting_time_book
where to_char(wpa_timestamp, 'YYYY') = '2010'

gives me
ORA-01847: day of month must be between 1 and last day of month. The to_char(...) in the where clause that does not work is exactly the same as the to_char(...) in the select clause in both queries, also the first one that works fine.

The view_waiting_time_book is a quite complex view where two strings are concatenated and converted into a date as wpa_timestamp, but it still works fins to select with a to_char from it in the first example.

Please help! The version of Oracle is Oracle Database 10g Enterprise Edition Release 10.2.0.5.0 - 64bit Production.

Thanks!
::Petter Received on Tue Oct 19 2010 - 11:29:16 CDT