sorry for duplicating this message (ORA-01422 when altering table)

i posted the following message in another gorup - but maybe someone here knows the answer ?

thanks !

oracle 9.2.0.1

i'm trying to add not null coulmn to existing table.

i'm running :
ALTER TABLE tbtat ADD (TCLASSID INTEGER default 22 not null);

 and i'm getting :
ORA-01422: exact fetch returns more than requested number of rows

any idea why ?