-440 Query UNTERMINATED after indefinite response

I trying to get measurement From the Power Supply Agilent E3634 but I become this error (-440 Query UNTERMINATED after indefinite response) 

This is my Code:

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using System.Threading;

using System.Management;

using System.IO.Ports;

using System.IO;

namespace ConsoleApplication1

{

class PortDataReceived

{

public static void Main()

{

SerialPort mySerialPort = new SerialPort("COM1");

mySerialPort.BaudRate = 9600;

mySerialPort.Parity = Parity.None;

mySerialPort.StopBits = StopBits.Two;

mySerialPort.DataBits = 8;

mySerialPort.Handshake = Handshake.None;

mySerialPort.RtsEnable = true;

mySerialPort.ReadTimeout = 1000;

mySerialPort.Open();

mySerialPort.WriteLine("MEAS:VOLT?");

Thread.Sleep(750);

string voltagevalue = mySerialPort.ReadExisting();

Console.WriteLine(voltagevalue);

mySerialPort.Close();

}

}

}