Object reference not set to an instance of an object.

We have a applicaton which, when our root web application pool is running .net 2 integrated, works fine.  As soon as I change it to .net 4 the application crashes. We actually have several doing that.  Here is the error...

Server Error in '/' Application.
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Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.  

Stack Trace:


[NullReferenceException: Object reference not set to an instance of an object.]
   ImageViewerHandler.ImageViewerHandler.ProcessRequest(HttpContext context) +226
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +596
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +265

Can anyone tell me what the object is that .net 4 is looking for and cannot find? I can't find anything!

Any help is greatly appeciated.

thanks
terri