Import from Google Sheets to an existing Project - can't import data into existing fields

I have a YouTrack project and I am trying to import external data into that project using the Google Sheets import process.

I have data that I'd like to import into custom fields that already exist in the YouTrack project. None of these fields use reserved names.

I can import the data from the Google Sheets, but rather than importing the data for a custom fields into the existing field, it creates a new field; for example rather than importing the data from the 'Product' column in the spreadsheet to the ‘Product’ field in the YouTrack project, it creates a new field named ‘Product_0’ and imports the data there. 

What am I doing wrong?