I am trying to run the following query and I get the error listed below. I cannot disable the Not NULL contraint, and I have verified that neither of the tables DFU tables contain null values in the DISC column. Any ideas would be apprecited.

update dfu a
set a.disc = (select b.disc
where a.dmdunit = b.dmdunit
and a.dmdgroup = b.dmdgroup
and a.loc = b.loc)

The following error has occurred:

ORA-01407: cannot update ("STSC"."DFU"."DISC") to NULL