Configuration (CTO) using the Action 'SET' with configured parts

Using a Configuration Combination table to return the component part number and this works OK unless the part number that it returns is a Configured Part. The supply code on the Customer order line is DOP and when I add the configured part as a component it works fine, but when I have a ‘place holder’ part as the componet part (ie just a dummy purchased part part), and then I use a configuration combination to ‘SET’ the componet part to a differant part number, it fails when the part is the configured part.  The error message I get is ‘Unable to process due to error action ‘SET’  Component part XX-XXX must not be a configured part.