how can i get the dropdown list value in gridview..

already i wrote the code is given below

protected void DropDownList_SelectedIndexchanged(object sender, EventArgs e)
{
// if (!Page.IsPostBack)
{
Session["uPrice"] = ((DropDownList)sender).SelectedValue;
}
}

but i get the error is "object null reference" the session variable is empty..

how can i get the dropdownlist selected value...

anybody pls help me