Bad Bind Variable Error In Oracle Forms 10G

declare
v1 varchar2(20);
v2 varchar2(20);

begin

select username , password
into v1 , v2
from users
where username=:users.username and password=:users.password;
go_block('home');
exception
when no_data_found then
set_item_property('msg',visible,property_true);
:users.username:=null;
:users.password:=null;
go_item('users.username');
end;