未将对象引用设置到对象的实例

protected void ibtnBuy_Click(object sender, ImageClickEventArgs e)
{
OrderBusiness ob = new OrderBusiness();
Order o = new Order();
o.PID=Convert.ToInt32(_pid);
o.BID = ((Users)Session["buyer"]).Userid;//说我这句有问题
o.SellerID=Convert.ToInt32(hfSellerID.Value);
o.Price=Convert.ToDouble(lblPrice.Text);
o.Quantity=Convert.ToInt32(lblQuantity.Text);
o.OrderDate = DateTime.Now.ToString();
o.OrderStatus=0;
if (ob.AddOrder(o) > 0)
{
Response.Write(" <script type='text/javascript'> alert('添加成功'); </script> ");
}
else
{
Response.Write(" <script type='text/javascript'> alert('添加失败'); </script> ");
}
}
NullReferenceException: 未将对象引用设置到对象的实例。] ShowProduct.ibtnBuy_Click(Object sender, ImageClickEventArgs e) in e:\学习\大学课程\3下（.NET实训）李绍华\小组网站\dessertWebSite\HoneySweet\ShowProduct.aspx.cs:80 System.Web.UI.WebControls.ImageButton.OnClick(ImageClickEventArgs e) +108 System.Web.UI.WebControls.ImageButton.RaisePostBackEvent(String eventArgument) +118 System.Web.UI.WebControls.ImageButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10 System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13 System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +36 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1565