How to Fix the "undefined method `upcase' for nil:NilClass:" Error in Xero

This error indicates that there's an issue with a data field in the invoice, such as a null or improperly formatted value, that is preventing it from syncing correctly with Xero. This often happens with the address field.

Since invoices cannot be edited, we recommend manually creating these invoices in Xero as they will not sync over unless this error is resolved.