DTS Script Runtime Error in SSIS

I am running a package in SSIS in VS 2013 BIDS. There is a script that is part of a Trigger API task in the SSIS project. When the task is run the following error is shown:

at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
   at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.RuntimeType.InvokeMember(String name, BindingFlags bindingFlags, Binder binder, Object target, Object[] providedArgs, ParameterModifier[] modifiers, CultureInfo culture, String[] namedParams)
   at Microsoft.SqlServer.Dts.Tasks.ScriptTask.VSTATaskScriptingEngine.ExecuteScript()

The C# script is:

       public void Main()
        {
            var webGet = new HtmlWeb();
            var document = webGet.Load(Dts.Variables["User::processUrl"].Value.ToString());
            var status = document.DocumentNode.SelectSingleNode("//div[@class='alert alert-success alert-dismissible']");
           // MessageBox.Show(status.InnerText.ToString(), "Error Title", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);

            if (status.InnerText.Trim().Contains( Convert.ToString("Query ran successfully.")) )
            {
                Dts.TaskResult = (int)ScriptResults.Success;
            }
            else
            {
                Dts.TaskResult = (int)ScriptResults.Failure;
            }
        }

The Namespaces used are (.NET 4.5 references)

using System;
using System.Data;
using Microsoft.SqlServer.Dts.Runtime;
using System.Windows.Forms;
using System.IO;
using System.Net;
using HtmlAgilityPack;
using System.Linq;

The error message fires on the line: 

var webGet = new HtmlWeb();

I am trying to decipher the error message to use in troubleshoot the script. Any help would be appreciated.