Billing Request can't create a subscription request (not supported by schema)

[RobertCadwaladr]

I am trying to create a billing request using the following API call:

const response = await client.billingRequests.create({
subscription_request: {
amount: '100',
currency: 'GBP',
day_of_month: '1',
interval_unit: 'monthly' as BillingRequestSubscriptionRequestIntervalUnit
}
});

return response.id

This should return the Billing Request ID but instead I get the following error:

errors: [
{
reason: 'request_type_not_supported_by_scheme',
message: 'Request type is not supported by scheme and/or consent_type.'
}
]

I'm not sure what part of this request doesn't match the schema, see: https://github.com/gocardless/gocardless-nodejs/blob/master/src/types/Types.ts#L983