Rosservice without arguments

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Hallo,

i want to establish a server without any arguments, so i call it like that: rosservice call /InputVoltage and just receive some output.

my code looks like that:

bool InputVoltage(ros_dc2412_upsd::InputVoltage::Response &req,
                  ros_dc2412_upsd::InputVoltage::Response &res)
{
  std::string string_var;
  ros::param::get("/string_a", string_var);

  DC2412 First(string_var);
  res.sum = First.InputVoltage();
  return true;
}


int main(int argc, char **argv)
{

  ros::init(argc, argv, "InputVoltage_server");
  ros::NodeHandle n;

  ros::ServiceServer service1 = n.advertiseService("InputVoltage", InputVoltage);


  ROS_INFO("Ready to deliver Inputvoltage");
  ros::spin();

  return 0;
}

my srv code:

---
int64 sum

if i run my code i recieve:

ERROR: Incompatible arguments to call service:

thanks for helping