Error using new beta Import functionality with custom module

Error using new beta Import functionality with custom module

I'm getting an error using the new beta Import functionality in Desk to get some data into a Custom Module.
My custom module is called "Senders" and is linked to "Accounts" with a Lookup field.
In the csv file for the import there is a column where I have populated the Zoho ID for the associated Account record. What I'm hoping to achieve on import is creation of records in the custom module that are properly associated with the correct Account in Desk.
I'm mapping the AccountID column from the csv to the Account Name > AccountId lookup field in the custom module.
The import fails for all records with the following error in the log:
  1. {"errorCode":"INTERNAL_SERVER_ERROR","message":"An internal server error occurred while performing this operation."}
Is there something I'm doing incorrectly, or is this a bug in the new import functionality?