Edge case perhaps, but a non-complete postcode from/to will throw an array error for users, rather than a notice stating they need to full in their details:
{"fieldErrors":[{"code":"INVALID_ARGUMENT","message":"frompostalcode is required","field":"frompostalcode"},{"code":"INVALID_ARGUMENT","message":"topostalcode is required","field":"topostalcode"}]}
Edge case perhaps, but a non-complete postcode from/to will throw an array error for users, rather than a notice stating they need to full in their details: