How to return a specific value from a drop down selection with Item Rules

Summary

Unable to select a return value in the target attribute based on a conditon of another attribute

Content

Hello, 

I need to have the MRP MPS Planning attribute change to a text value based on the information in the Item Status attribute. 

In the return value section of my rule set, I can select the correct target attribute, but I am not able to select the exact value I need returned.  I am not able to select any value from the value set list for the attribute. See image attached 'blank drop down'.

Am I not able to see the list because the target attribute is a dropdown list? When I validate the rule with 'Not Planned' the error says 'Not Planned' is not a valid number. If I validate the rule without Not Planned in the return value, but with [Item].[MPS and MRP Planning].[Planning Method] in the return value there is no error.