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 table1
where 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 = 1
and 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?