Using external record source for dropdown

My application needs to reference an existing database for one of the columns. The main data table contains a VehicleId column (an identifier), the column to be referenced is of an integer data type, which is make available by importing through an extension and the ID column is set to data type Entity Reference (Integer). 

When creating the dropdown, I cannot select the value for the Source Identifier Attribute and got the following message: "The 'Source Identifier Attribute' property must be set to a valid attribute of the 'VEHICLE Record' Record."

How to make this work?

Have you tried leaving the ID column set to integer. And in the entity properties set the identifier to the Id attribute.
Tried it and got the error: 'VEHICLE' Identifier data type required instead of 'Integer'.
Forgot to mention I am on version
Brian's solution actually worked, the error I got was referring to the column in the main table needs to be changed from Integer to Vehicle Identifier.

Thanks Brian.