Create reference line in detail family

Hi,

I've attached a test project.  As I understand it, to create a reference line from the api, a model curve is created and then this is changed to a reference.

But if I attempt to create a model curve in a detail family (create a family using the template Metric Detail Item.rft prior to testing the attached)

then I get the exception 

Autodesk.Revit.Exceptions.InvalidOperationException was unhandled by user code
HResult=-2146233088
Message=The attempted operation is not permitted in this type of family.

How do I create a reference line in a detail family?  The User interface enables this.

Thanks in advance,

Jon