SQL question

I have table with the following definition:
create table table1(    account_no    int4,    start_date_tme    datetime    ....);

The table may contain null values for start_date_time.

When I run the following SQL query, it fails:
select * from table1where start_date_time::date >= '01/01/2000'::date and start_date_time::date <= '01/01/2001'::date;

I get error message 'Unable to convert null datetime to date.

It's fine if I run the same SQL query with added condition as follows:
select * from table1
>    where account_no = 1and start_date_time::date >= '01/01/2000'::date and start_date_time::date <=
'01/01/2001'::date;

Is this a bug? Or there's an logical explaination for this?