Mit program crasher efter denne kode

public void AllocateGlobalMem(string myString)
        {
            char[] teststring = myString.ToCharArray();
            IntPtr myHandle = Marshal.AllocHGlobal(myString.Length);
            Console.WriteLine("Global handle: "+Convert.ToString(myHandle));
            Marshal.Copy(teststring, 0, myHandle, teststring.Length);
            Console.WriteLine("Writing to memory");
            Console.WriteLine("Reading from memory");
           
            string mytext = Marshal.PtrToStringUni(myHandle);
            Console.WriteLine("String read from mem: "+mytext);
            Marshal.FreeHGlobal(myHandle);
        }

Herefter laver den en NullPointer exception i min main metode........