getting error must declare scalar variable

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
 <asp:ScriptManager ID="scm" runat="server"></asp:ScriptManager>
 <table>
    <tr>
        <td>
            <asp:DropDownList ID="ddldepartment" runat="server"
                onselectedindexchanged="ddldepartment_SelectedIndexChanged" AutoPostBack="true"></asp:DropDownList>
        </td>
    </tr>
    <tr>
         <td>
            <asp:GridView ID="gv" runat="server" DataKeyNames="empid"></asp:GridView>
        </td>
    </tr>
 </table>
</asp:Content>

public partial class GetDetailsbydropdown : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            CustomersBusiness cb = new CustomersBusiness();
            if (!IsPostBack)
            {
                ddldepartment.DataSource = cb.GetDepartment();
                ddldepartment.DataTextField = "departmentid";
                ddldepartment.DataValueField = "departmentname";
                ddldepartment.DataBind();
            }
        }

        protected void ddldepartment_SelectedIndexChanged(object sender, EventArgs e)
        {
            CustomersBusiness cb=new CustomersBusiness();
            Customers cs = new Customers();
            cs.departmentid = Convert.ToInt32(ddldepartment.SelectedItem.Text);
            gv.DataSource = cb.GetEmployeebyDepartment(cs);
            gv.DataBind();
        }
    }

in businesslogic.cs

        public List<customers> GetEmployeebyDepartment(Customers cs)
        {
            List<customers> lst = new List<customers>();
            SqlConnection con = new SqlConnection(constring);
            con.Open();
            string query = "select empid, empname, departmentid, joiningdate, locationid, salary, jobid, isworking where departmentid=@departmentid";
            cmd = new SqlCommand(query, con);
            dr = cmd.ExecuteReader();
            if (dr.HasRows)
            {
                while (dr.Read())
                {
                    Customers cst = new Customers();
                    cst.employeeid = (int)dr[0];
                    cst.empname = (string)dr[1];
                    cst.departmentid = (int)dr[2];
                    cst.joiningdate = (DateTime)dr[3];
                    cst.locationid = (int)dr[4];
                    cst.salary = (int)dr[5];
                    cst.jobid = (int)dr[6];
                    cst.isworking = (Boolean)dr[7];
                    lst.Add(cst);
                }
            }
            return lst;
        }