Assign value not null a null value
SQL> SQL> declare 2 myNumber number not null := 1; 3 another_number number; 4 begin 5 myNumber := 2; 6 myNumber := another_number; 7 end; 8 / declare * ERROR at line 1: ORA-06502: PL/SQL: numeric or value error ORA-06512: at line 6 SQL>