1, long non-empty column constraint can not be added

Issue：You can alter the table to include the NOT Nun., constraint on the EMP_IMAGE column.

15:12:34 SQL> create table emp_details(
           2  emp_id number not null,
           3  emp_name varchar(40) not null,
           4  emp_image long);
Table created
Executed in 0.262 seconds

16:37:38 SQL> alter table emp_details modify emp_image not null;
Table altered
Executed in 0.526 second

doubt:

　　The answer saying this is wrong, but did not give an explanation;