materialized view with refresh on commit

1. I have a view:

     create view ni_v as select * from cartis union select * from ni_c;

2.  i try to create materialized view

create materialized view ni_mv
refresh on commit
as
    select account
    ,sum(mataz)                    execution_mtz
     from ni_v
     where account between '200000' and '299999' -- to quick the query
     group by account

3. I receive  

     ORA-12054: cannot set the ON COMMIT refresh attribute for the materialized view

4. i built the above mv but with BUILD DEFERRED

5. result  of EXECUTE DBMS_MVIEW.EXPLAIN_MVIEW ('ni_mv') is :

     MV with UNION ALL in a view is too complex