variable name in plpgsql

It seems that one cannot use the variable name that is the same as the
column name of a relation like in the following function

------
create or replace function bla() returns void language plpgsql as $$
declare
    email varchar;
begin
    select email into email from pass where id=1;
    raise notice 'pass is %',email;
end;
$$
------
I get null value email and postgres 8.4.4 does not warn nor raise
error on creating;
I know for a fact that email must not be null; it's not null when I
rename the variable
email to something else.

As far as I can see this isn't documented anywhere.  Is this because
it's common/insider knowledge?  Can someone confirm?

Thanks