Display Detail of data from database based on data in session list like comparison

Question

I have get error - please can anyone help?

Object reference not set to an instance of an object.

My code:

public ActionResult Detail()
{
    //this list stored data selection to Compare Between them //
    List<com> list = (List<com>)Session["com"];

    // the specsat List to Display detial of data  based on data in <list> 
    List<specsat> spe = new List<specsat>();

    foreach (var co in list)
    {
       spe.Add(new specsat(db.Specs.Find(co.Ct.id)));
    }

    return View(spe.ToList());
}