Strange error in update app

[McMillan0520]

well, I am building an update application for my project,and the way i'm checking if there are new updates is by a webBrowser. Here is the function that checks for updates (it's called by the OnDocumentCompleted event):

void Check()
		{
			
			string content = webBrowser1.DocumentText;
			string[] checking = content.Split('|');
			string data = checking[1];
			if (data != "_MTL_VERSIONS_EQUAL_")
			{
				ProcessData(data);
				
				if(MessageBox.Show("Znaleziono w internecie poprawkę, z wersji aktualnej - "+version+", na "+upVersion+". Czy chcesz ją pobrać?", "Łatki - Multitool Update", MessageBoxButtons.YesNo)==DialogResult.Yes)
				{
					
					MessageBox.Show("Za chwilę rozpocznie się pobieranie i instalacja łatki.", "Łatki - Multitool Update", MessageBoxButtons.OK, MessageBoxIcon.Information);
					RegistryKey rejestr=Registry.CurrentUser.OpenSubKey("software\\multitool\\UpDate",true);
					fPath = Convert.ToString(rejestr.GetValue("DownPath"));//tymczasowa zmiana
					button2.Text="Anuluj";
					thr=new Thread(new ThreadStart(Download));
					thr.Start();
				}
				else
				{
					Application.Exit();
				}
				
			}
			else
			{
				MessageBox.Show("Nie znaleziono żadnych poprawek.","Łatki - Multitool Update", MessageBoxButtons.OK, MessageBoxIcon.Information);
				Application.Exit();
			}
			
		}
		
		
		
		void WebBrowser1DocumentCompleted(object sender, System.Windows.Forms.WebBrowserDocumentCompletedEventArgs e)
		{
			thr=new Thread(new ThreadStart(Check));
			thr.Start();
		}

(sorry the messages are in Polish)
and it gives out an error:
An unhandled exception of type 'System.InvalidCastException' occurred in System.Windows.Forms.dll

Additional information: Specified cast is not valid.

The error is in the line:

string content = webBrowser1.DocumentText;

The documentText value is string so why is it giving out the error?
Please answer quickly.

[McMillan0520]

Right, I fixed it. The problem was in a completely different part of the code. I used the same thread reference twice.