Hey ashead,
Regenerate the Entity, Customer, and Referral Source Entity tables (in that order). To regenerate a table go to Customize > Tables/Fields > click on the table name (e.g. Entity) > then click Finished Editing Fields. Rinse and repeat for the other tables, then check your screen again. *Note: When you are in the Referral Source Entity table, are any of your fields marked as Unique?
If the problem persists, edit your screen, and edit the Referral Source Entity field. Go to 'Step 4 of 6: Dropdown Choices' and ensure the Select Field value is not blank; if it is then click the dropdown and select the appropriate field. Hope this helps!
- Ashur