ASP XMLHTTP request fails

Event code: 3005 
Event message: An unhandled exception has occurred. 
Event time: 02/11/2011 14:55:42 
Event time (UTC): 02/11/2011 14:55:42 
Event ID: 4e550d910b934d2781707701f833e18e 
Event sequence: 39 
Event occurrence: 1 
Event detail code: 0 
 
Application information: 
    Application domain: /LM/W3SVC/1/ROOT-2-129647191892089824 
    Trust level: Full 
    Application Virtual Path: / 
    Application Path: C:\inetpub\wwwroot\ScirraNew\ 
    Machine name: TOM-PC 
 
Process information: 
    Process ID: 7980 
    Process name: w3wp.exe 
    Account name: NT AUTHORITY\NETWORK SERVICE 
 
Exception information: 
    Exception type: ArgumentNullException 
    Exception message: Value cannot be null.
Parameter name: String
   at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)
   at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info)
   at System.Int32.Parse(String s)
   at forumalerts.ProcessRequest(HttpContext context) in c:\inetpub\wwwroot\ScirraNew\Handlers\forumalerts.ashx:line 13
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

 
 
Request information: 
    Request URL: http://127.0.0.1/handlers/forumalerts.ashx 
    Request path: /handlers/forumalerts.ashx 
    User host address: 127.0.0.1 
    User:  
    Is authenticated: False 
    Authentication Type:  
    Thread account name: NT AUTHORITY\NETWORK SERVICE 
 
Thread information: 
    Thread ID: 39 
    Thread account name: NT AUTHORITY\NETWORK SERVICE 
    Is impersonating: True 
    Stack trace:    at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)
   at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info)
   at System.Int32.Parse(String s)
   at forumalerts.ProcessRequest(HttpContext context) in c:\inetpub\wwwroot\ScirraNew\Handlers\forumalerts.ashx:line 13
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
 
 
Custom event details: 

    // Get various params
    int TopicID = int.Parse(context.Request.Form["topicID"]);
    int ThreadID = int.Parse(context.Request.Form["threadID"]);
    bool SecretOK = context.Request.Form["secret"].Trim() == "123";