I can't figure out what is causing this error.  As always, thanks in advance!

Timestamp: 5/9/2008 3:21:02 AM
Message: HandlingInstanceID: 4302c641-a050-45be-b69c-fc6bc65bea37
An exception of type 'System.Web.HttpUnhandledException' occurred and was caught.
---------------------------------------------------------------------------------
05/08/2008 20:21:02
Type : System.Web.HttpUnhandledException, System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
Message : Exception of type 'System.Web.HttpUnhandledException' was thrown.
Source : System.Web
Help link :
ErrorCode : -2147467259
Data : System.Collections.ListDictionaryInternal
TargetSite : Boolean HandleError(System.Exception)
Stack Trace :    at System.Web.UI.Page.HandleError(Exception e)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at ASP.store_paypalipnhandler_aspx.ProcessRequest(HttpContext context) in c:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\395c055d\bed154\App_Web_llmoyqwn.46.cs:line 0
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Additional Info:

MachineName : CT3459
TimeStamp : 5/9/2008 3:21:02 AM
FullName : Microsoft.Practices.EnterpriseLibrary.ExceptionHandling, Version=2.0.0.0, Culture=neutral, PublicKeyToken=null
AppDomainName : /LM/W3SVC/1/Root-1-128547757234038750
ThreadIdentity :
WindowsIdentity : NT AUTHORITY\NETWORK SERVICE
Inner Exception
---------------
Type : System.Collections.Generic.KeyNotFoundException, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
Message : The given key was not present in the dictionary.
Source : mscorlib
Help link :
Data : System.Collections.ListDictionaryInternal
TargetSite : Void ThrowKeyNotFoundException()
Stack Trace :    at System.ThrowHelper.ThrowKeyNotFoundException()
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at PaypalIPNHandler.Page_Load(Object sender, EventArgs e) in c:\AKDSC\web\Store\PaypalIPNHandler.aspx.cs:line 46
   at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e)
   at System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e)
   at System.Web.UI.Control.OnLoad(EventArgs e)
   at System.Web.UI.Control.LoadRecursive()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)


Category: General
Priority: 0
EventId: 100
Severity: Error
Title:Enterprise Library Exception Handling
Machine: CT3459
Application Domain: /LM/W3SVC/1/Root-1-128547757234038750
Process Id: 13012
Process Name: c:\windows\system32\inetsrv\w3wp.exe
Win32 Thread Id: 56228
Thread Name:
Extended Properties: Url - http://www.ashlynkray.com/Store/PaypalIPNHandler.aspx
RawUrl - /Store/PaypalIPNHandler.aspx
UseAgent -
UrlReferrer -
IPAddress - 66.211.170.66

Line 46 is string orderNumber = values["custom"];

It says the the name value pairs that it received from PayPal didn't have name "custom". The order submitted from DotShoppingCart to PayPal should always contain that name. Is it possible that there was a non-DotShoppingCart payment transaction going through your PayPal account?

The order number is automatically determined, so, I don't know where that is coming from.  It doesn't look like anyone is buying anything, because no orders have gone through today and I have received 5 of these error emails.  All from the same IP address though, so, any ideas?