Assuming arguments are strings leads to crash

[joelanman]

For example if a prototype kit user doesn't have an API key, they get this error

Cannot read properties of undefined (reading 'substring')

Because lines like this assume string:

https://github.com/alphagov/notifications-node-client/blob/main/client/api_client.js#L20

A more helpful error would be something like 'API key is required'