Updating data from view model mvc5 (EF6)

Question

I have a problem when I Update the database from model! I have an interface and I identify an Edit Function and I created a class to implement this function as follow:

public void Edit(byte? id)
{
       var entity = getByID(id);
       UnitOfWork.DbContext.Entry(entity).State = EntityState.Modified;
       UnitOfWork.Commit();
}

In the controller I called this function as follow:

[HttpPost]
public ActionResult Edit(BackendModuleViewModel model)
{
       var entity = model.ConvertToEntity(model);
       _backendModuleRepository.Edit(entity.BNM_ID);
        return RedirectToAction("Index");
}

When I run the project it tells me that the primary key is null !

The function (getByID) in Edit that located in a repository is implemented as follow:

public BackendModule getByID(byte? id) 
{ 
      var entity = DbSet.FirstOrDefault(x => x.BNM_ID == id); 
      return entity; 
}