Importing invoices fails due to missing validator

I am trying to migrate from a different online accounting service. I have downloaded the template for invoices, customers and items.

I am able to import my customers and items, but am having troubles with the invoices.

When I have a invoice in the template, filling in the invoice details and the invoice_items details, I receive an error that:

Method
Illuminate\Validation\Validator::validateQuantity does not exist.

Any assistance here would be awesome. I have tried this in the app, on Chrome and on Edge.