PPI error

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PPI error

I am trying PPI
CREATE SET TABLE Employee
(Employee_Number INTEGER NOT NULL
,Location_Number INTEGER
,Dept_Number INTEGER
,Emp_Mgr_Number INTEGER
,Job_Code INTEGER
,Last_Name CHAR(20)
,First_Name VARCHAR(20)
,Salary_Amount DECIMAL(10,2))
UNIQUE PRIMARY INDEX (Employee_Number)
PARTITION BY Location_Number;

It is throwing an error ----Location_Number shud be a part of PI for its uniqueness....

I have read that Location_Number should be a part of PI to make it UNIQUE....
But not able to figure out why....
Can anyone please explain why Location_Number needs to be included although Employee_Number is unique....
Please provide some example....