asp.net unchecked checkbox pass not value as NULL

I'am trying to insert some data from ASP.NET DropDownList and CheckBox to database if the Checkbox is checked I want to insert DropDownList value if not checked I want to pass null. But in my table it inserts 0 not NULL How can I pass null instead of zero (0) if it is not checked? Her is my code:

if (CheckBoxMorning.Checked == true)
   {
     SqlParameter paramMorning = new SqlParameter("@Morning", DropDownMorning.SelectedValue);
     cmd.Parameters.Add(paramMorning);
    }

     else
    {
      SqlParameter paramMorning = new SqlParameter("@Morning", CheckBoxMorning.Checked);
      cmd.Parameters.Add(paramMorning);
     }

Thank you in advance