I try to create the following materialized view:

create materialized view mw_join_fast_on_com1
storage (initial 10K next 10K pctincrease 0)
build immediate
refresh fast on demand
as
select e.sal, d.deptno, e.rowid as "erid", d.rowid as "drid",
e.job
from dept d, emp e
where d.deptno=20 and
d.deptno=e.deptno;

but i receive the error
ORA-12015: cannot create a fast refresh snapshot from a complex query

This statement works well if I use "refresh complete" or if I use "refresh fast" but without where "d.deptno=20" condition.
What does it mean? How can I solve my problem?
Thanks
Vincenzo