Object Reference not set to an instance of an object - Cannot see what's wrong

Question

Trying to do this edit page with asp.net MVC c# and getting the "NullReferenceException: Object reference not set to an instance of an object." error, but cannot see for the life of me what's actually wrong with the code, so a second pair of eyes would be hugely helpful :)

[HttpPost]
public ActionResult Edit(EquipmentIndexViewModel viewModel)
{
    if (ModelState.IsValid)
    {
        var Equipment = TheEquipment.tblEquipments.Find(viewModel.EquipmentId);
        Equipment.EquipmentID = viewModel.EquipmentId;
        Equipment.EquipmentName = viewModel.EquipmentName;
        Equipment.EquipmentICTVisible = viewModel.ICTVisibile;
        Equipment.EquipmentPublicVisible = viewModel.PublicVisible;

        TheEquipment.SaveChanges();

        return RedirectToAction("EquipmentList");
    }
    return View(viewModel);
}

The stack trace is pointing to the following line:

Equipment.EquipmentID = viewModel.EquipmentId;

but it must be finding the ID find as Controller/Edit/ID is working fine, or seemingly anyway

any help would be appreciated!

Thanks