Payment direct debit impossible

I've been trying to configure direct debit payments for months, but it's impossible to do so because of a bug in webapi.voo.be:
Request sent:

{"ibanAccount":"BESomeIbanAccount","firstName":"someFirstName","lastName":"someLastName"}

Response from API: 422 with body:

{"messages":[  {"code":"LastName","description":"The LastName field is required.","correlationId":""},  {"code":"FirstName","description":"The FirstName field is required.","correlationId":""},  {"code":"IbanAccount","description":"The IbanAccount field is required.","correlationId":""}]}

Note: if the 'code' values should match the fieldnames in the request body, they're not. The casing is different.