Forgein key constraint in mysql. I cannot fix it? -

i created following table in mysql follow:

create table employee (   fname    varchar(15) not null,    minit    varchar(1),   lname    varchar(15) not null,   ssn      char(9),   bdate    date,   address  varchar(50),   sex      char,   salary   decimal(10,2),   superssn char(9),   dno      integer(4),   primary key (ssn),   foreign key (superssn) references employee(ssn) ); 

then tried insert following information:

insert employee values ("bob","b","bender","666666600", "1968-04-17","8794 garfield, chicago, il","m","96000.00","null","8"); 

sadly, got error:

error code: 1452. cannot add or update child row: foreign key constraint  fails (`test`.`employee`, constraint `employee_ibfk_1` foreign key  (`superssn`) references `employee` (`ssn`)) 

i tried alter table superssn can null no solution working.

alter table employee modify column superssn char(9) default null; 

"null" not same null. remove quotes.