Replication :: MV Refresh Error

During the MV refresh we are getting the following error and not able to fix it .

SQL> EXECUTE DBMS_MVIEW.REFRESH('EMP','F');
BEGIN DBMS_MVIEW.REFRESH('EMP','F'); END;

*
ERROR at line 1:
ORA-12008: error in materialized view refresh path
ORA-01400: cannot insert NULL into ("XYZ"."EMP"."DESCRIPTION")
ORA-06512: at "SYS.DBMS_SNAPSHOT", line 2251
ORA-06512: at "SYS.DBMS_SNAPSHOT", line 2457
ORA-06512: at "SYS.DBMS_SNAPSHOT", line 2426
ORA-06512: at line 1

The above column:- Emp.Description is a 'CLOB NOT NULL' .

We have created the MV using the below script

create materialized view EMP refresh on demand for update as select *from ABC.EMP@DBLINK;