Cannot access ParentEntity for an Inheritance in a script

I'm writing a VBscript function to determine the ultimate parent entity for a LDM inheritance. It falls over at the first step, because PD doesn't allow me to access the parent entity for the Inheritance as an object

Here's a simple Get Method to clone this property into a text computed extended attribute:

Function %Get%(obj)

  %Get% = obj.ParentEntity

End Function

The extended attribute is populated with a string such as 'Entity : Fred', not with an object.