Using Models folder with webforms when using Windows Authentication

I'm running Visual Studio 2013.4.  I'm trying to have a 'Models' folder in my webforms project where I put my POCO classes for EF.  It works fine if I use the 'Individual accounts' authentication option but when I use Windows Authentication, I have a problem.  In my code behind, I try including the Models namespace (such as this:  using WebApplication1.Models;).  I'm given the following:
The type or namespace name 'Models' does not exist in the namespace 'WebApplication1' (are you missing an assembly reference?).  
It works fine when I'm using the individual accounts authentication option but I need it to be authenticated via Active Directory.  I've tried multiple variations of the project (hybrid webforms/mvc/webapi), different auth options.  If I use a strict MVC project with Windows Authentication, it works fine but some of what I need to use still requires webforms for now.
Any ideas?  Is there a way to u ...