Workflow Issue: Cannot set Operator "Set to" to entity's primary field.

Hello, 

I've created a custom entity entitled 'Supplier Purchase Order'. It's primary field is called 'name'. 

I'm trying to use the "SETWORDTEMPLATE" action in a Workflow on this entity, however when setting the Target the option to select {Supplier Purchase Order(Supplier Purchase Order)} does not present itself like it does when I reference other entities. 

Here's a screenshot: 

Furthermore, if I select {Created By(Supplier Purchase Order)} or any of the others the workflow throws the following error:

Business Process Error: Creating Entity with an invalid parent. Entity: Annotation, ReferencingAttribute:objectid\

Error code: 0x80040265

Can anyone shed some light on what I could be doing wrong?

Much appreciated!