IIS NullReferenceException when deployed but not in Dev Server

Question

I have developed an IIS Application using C# code behind ASPX pages with entity framework for database access. When I run it in the Visual Studio Development Server it works fine but if I either Publish it to IIS or run it on the Local IIS Web Server from Visual Studio (which is pretty much the same as Publishing I guess) I get

NullReferenceException: Object reference not set to an instance of an object.]
   System.Web.Hosting.RecyclableCharBuffer.Append(String s) +15
   System.Web.Hosting.ISAPIWorkerRequest.SendUnknownResponseHeader(String name, String value) +93
   System.Web.HttpResponse.WriteHeaders() +233
   System.Web.HttpResponse.Flush(Boolean finalFlush) +219
   System.Web.HttpRuntime.FinishRequest(HttpWorkerRequest wr, HttpContext context, Exception e) +127

This Exception is thrown after my Home.aspx has finished it's Page Load. Some simple ASPX pages work ok. Setting a NullReferenceException break point just drops into disassembly.

I am stuck for ideas about how to track this issue down. Any thoughts???

Answer 1

Finally tracked this down. Some of out template code in the master page was doing

Response.AddHeader("X-UA-Compatible", (String)Application["X-UA-Compatible"]);

when Published Application["X-UA-Compatible"] was null.

It's worth knowing that having nulls in Response.AddHeader() causes this kind of problem rather than handling the null more gracefully.