Partitioning not working need primary key?

[newbie Shai]

Dear All,
We are trying some partitioning as below.

ALTER TABLE tblreceipt PARTITION BY RANGE (MONTH(receiptDate))
(
PARTITION p1 VALUES LESS THAN (TO_DAYS('2012-10-20')),
PARTITION p2 VALUES LESS THAN (TO_DAYS('2012-10-21')),
PARTITION p3 VALUES LESS THAN (TO_DAYS('2012-10-22'))
);

but end up with this Error Code: 1503
A PRIMARY KEY must include all columns in the table's partitioning function