Error when trying to update a custom field on the Offer entity

Summary

Error when trying to update a custom field on the Offer entity

Content

I have a custom field on the Offer entity that I want to update.  I used the wizard to create a merge import that has three columns.  My search columns are Number and Application,Number.  For both of them the only box I checked in the Behavior section is "Use the value to search or lookup".  The third column is the custom one whose value I want to update.  For that column none of the boxes are checked in the Behavior section. 

I'm getting this error.  "The create (only) operation was selected for this instruction, however the entity was found in the product.;"