Bad Idea Challenge #3

This looks like okay code, doesn’t it?

   public IEnumerable<County> GetCounties(int stateId)
   {
       using (var ef = new TopologyEntities())
       {
           return ef.Counties.Where(c => c.stateId == stateId);
       }            
   }

After all, it’s using the using pattern with the IDisposable-implementing EF data context.   Isn’t that what you’re supposed to do?

But:  the following code will throw an error. 

       var counties = GetCounties(10);
       var count = counties.Count();

Do you see why?

(For bonus points) Which line of this code will throw the exception? Why?