Request function headers can't accept null value for any of its props

If any of my headers has a value other than a string, the headers in my request object are set to empty {}.
As per the documentation here, the headers keys and values should be strings.
Unfortunately, I am trying to send a post request to an API that is expecting a particular header to be null for one of the requests. Tried sending empty string instead, but the API fails to process the request.
Any way around this?