SQL & PL/SQL :: Update Column To Consecutive Number?

create table test_table(
rn varchar2(10),
col1 varchar2(10),
col2 varchar2(10),

[code]...

I want update col1 whis is null to max(col1) ++ in a row, order by cr_date like
1,1,20110102
2,2,20110101
3,null,20110105 => 3,5,20110105 because this row is after 20110103
4,3,20110104
5,null,20110103 => 5,4,20110103 because this row is before 20110105

update test_table
set col1 = (select max(col1) from test_table) + rownum
where col1 is null;

this gives ora-00933