How to use getblocktemplate method with Bitcoinlib Library

I am using bitcoinlib to communicate with bitcoin-qt

This is the code i tried:

 Dim BitcoinService As Services.RpcServices.RpcService.IRpcService = New Services.Coins.Bitcoin.BitcoinService("http://localhost:8332", "Username", "Password", "wallet password", 200)

for getblocktemplate this is the call

   Dim seg As JToken = "{""rules"": [""segwit""]}"

        RichTextBox5.Text = BitcoinService.GetBlockTemplate(seg).PreviousBlockHash

It gives me this error

BitcoinLib.ExceptionHandling.Rpc.RpcInternalServerErrorException
  HResult=0x80131500
  Message=JSON value is not an object as expected
  Source=BitcoinLib
  Arborescence des appels de procédure :
   à BitcoinLib.RPC.Connector.RpcConnector.MakeRequest[T](RpcMethods rpcMethod, Object[] parameters)
   à BitcoinLib.Services.CoinService.GetBlockTemplate(Object[] parameters)
   à WindowsApp3.Form1.Button2_Click(Object sender, EventArgs e) dans C:\Users\Hama\source\repos\WindowsApp3\WindowsApp3\Form1.vb :ligne 76
   à System.Windows.Forms.Control.OnClick(EventArgs e)
   à System.Windows.Forms.Button.OnClick(EventArgs e)
   à System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
   à System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   à System.Windows.Forms.Control.WndProc(Message& m)
   à System.Windows.Forms.ButtonBase.WndProc(Message& m)
   à System.Windows.Forms.Button.WndProc(Message& m)
   à System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   à System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   à System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   à System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   à System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
   à System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   à System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   à Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.OnRun()
   à Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.DoApplicationModel()
   à Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.Run(String[] commandLine)
   à WindowsApp3.My.MyApplication.Main(String[] Args) dans  :ligne 81

  Cette exception a été levée à l'origine dans cette pile des appels :
    System.Net.HttpWebRequest.GetResponse()
    BitcoinLib.RPC.Connector.RpcConnector.MakeRequest<T>(BitcoinLib.RPC.Specifications.RpcMethods, object[])

Exception interne 1 :
WebException : Le serveur distant a retourné une erreur : (500) Erreur interne du serveur.

I dont understand what i am doing wrong, it works with other methods, i think it's a json problem

Help is appreciated, thank you

Try

 Dim seg As JToken = JToken.Parse("{""rules"": [""segwit""]}")

because assigning a string to a JToken doesn't parse the string as json, it just makes a json string that has the given string as its value