Why am I getting the error message "id value of incorrect type" in the NPSP Data Import Process?

I've been loading donation data from a spreadsheet every month or so using the NPSP Import Process, but this time I get the error message "Channel: id value of incorrect type: Team USA" after the initial import (upload) into the NPSP Data Import custom object. "Channel" is a custom field based on a lookup relationship with the account object. I read in the account's Salesforce ID correctly and it gets converted to the correct channel: "Team USA". What is the id that has the wrong type?