code: func (fields *UserProfileCustomFields) UnmarshalJSON(b []byte) error {
  	// https://github.com/nlopes/slack/pull/298#discussion_r185159233
  	if string(b) == "[]" {
  		return nil
  	}
  	return json.Unmarshal(b, &fields.fields)
  }
docs: // UnmarshalJSON is the implementation of the json.Unmarshaler interface.