SQL & PL/SQL

Hi All
I have a Job running in Production and want to update END_DATE as NULL
I tried below command and giving error
begin
dbms_scheduler.set_attribute (
name => 'MY_JOB',
attribute => 'END_DATE',
value => null);
end;
Error report:
ORA-06550: line 2, column 1:
PLS-00307: too many declarations of 'SET_ATTRIBUTE' match this call
ORA-06550: line 2, column 1:
PL/SQL: Statement ignored
06550. 00000 - "line %s, column %s:\n%s"
*Cause: Usually a PL/SQL compilation error.
*Action:

Thanks for your help