Unable to cast object of type 'System.Int32' to type 'System.String'.

protected void btnAddToCart_Click(object sender, EventArgs e)
        {

            if (Session["countCart"] != null)
            {
                Session["countCart"] = (int)Session["countCart"]+1;
                lblCount.Text = (string)Session["countCart"];
            }
            else
            {
                Session["countCart"] = 0;
                lblCount.Text = (string)Session["countCart"];
            }
        }