create MV error - PK not found when using 'with rowid' in log

Hi can anyone explain this error:

I run the following commands in a script

create materialized view log on dnh_table with rowid

create materialized view mv_dnh_table
tablespace small_tables
refresh fast
on demand
as select * from dnh_table

and then get the following error on the last stmt

ORA-12014: table 'DNH_TABLE' does not contain a primary key constraint

I know the table doesn't have a PK, that's why I'm using 'with rowid' when I create the log

I'm running Oracle 8.1.7 on SUN (btw, this is a simplified version so please don't come back with why bother using a MV to do this etc, etc)

cheers
  nigel

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