insert null into database if users didnt select any item from dropdownlist

Hi there

according to below thread

http://www.aspforums.net/Threads/176728/insert-null-in-data-base-if-user-dont-select-Item-from-dropdownlist/?s=2#Replies

I can solve my problem but when I wrote this code in other page with different SP It didn't worked correctly I change SP

in new SP I used INSERT code instead of UPDATE

now when users didn't select Item from ddl3 it insert 'please select' in table these are my code

protected void ImageButton2_Click1(object sender, ImageClickEventArgs e)
   {
   
  
       SqlCommand _cmd = new SqlCommand("insertSreg", _cn);
       _cmd.CommandType = CommandType.StoredProcedure;
       _cn.Open();

           _cmd.Parameters.AddWithValue("@Name", txtstore.Text);
           _cmd.Parameters.AddWithValue("@Aname", Txtname.Text);
  if (DDL3.SelectedItem.Value!= "0")
            {
           _cmd.Parameters.AddWithValue("@ShoppingCenter", DDL3.SelectedItem.Text);
 }
  

  
           _cmd.ExecuteNonQuery();
           Session["Message"] = true;
  
           Response.Redirect("register.aspx");
  
  

  
        
      
       _cn.Close();
       
   }

SP

ALTER procedure [dbo].[insertSreg]
@Name nvarchar(40),@Aname nvarchar(40),
@shoppingcenter nvarchar(30)=NULL,


as

begin
insert into S_register (Name,Aname
,Shoppingcenter)
values(@Name,@Aname
,@Shoppingcenter)



end

NOw what should i do?

AGAIN THANKS ALOT