Postgres 8 - problem: invalid input syntax for integer

Hi, In previous version di Postgres (7.2) I used this table:
CREATE TABLE tablename (id serial, field int1, field2 text);

Now this query work:
UPDATE tablename SET field1=''; (NOTE: implicit conversion to 0)
UPDATE tablename SET field2='';

(this cause of simple code-generation query - I don't know what's
field type)

Now in postgres 8 this don't work.
Why ?(ok, it's the ufficial documentation but I don't understand...
why? it's so comfortable!)
Can someone help me to create a CAST to re-use this feature?
Thank you! Mauro