HR_POSITION_API.UPDATE_POSITION

I'm trying to call the HR_POSITION_API.UPDATE_POSITION api to update a field in the Date Effective Name on a position but keep getting the following error:

'Error occured. Code = -20001. Msg = ORA-20001: The primary key specified is invalid

Cause:        The primary key values specified are invalid and do not exist in the schema. 

Action:        Check the primary key . Position no = 45850'

Has anyone else come across the same problem?  I believe this maybe caused by the parameters I am using to call the API, particularly around dates.  There are 3 input parameters that can be defined as inputs, p_effective_date, p_date_effective and p_date_end - does anyone have any advice on what values they should hold?  Any advice would be greatly appreciated.