null values in a view

Hi all,

How could I create a view that returns null values among all other values.
Here is a sample that i want to achieve:

CREATE VIEW view1 AS
SELECT
attribute1 as a1,
null as a2
FROM
table;

Now the problem is that I got an warning: column "a2" has type "unknown"
I know that I should define a data type for a field a2. But how?
This works with other values but not with nulls:

CREATE VIEW view1 AS
SELECT
attribute1 as a1,
text null as a2,
text 'test' as a3
FROM
table;

Thanks

-Lauri Kajan