Invalid cast exception. Linq to SQL

 private void Ent_Click(object sender, EventArgs e)
        {
            int id = Convert.ToInt32(dataGridView2[5, dataGridView2.CurrentRow.Index].Value);
            string s = "(4.5; 2.5)";
            Essense_of_Raschet esr = Data_context.GetTable<Essense_of_Raschet>().FirstOrDefault(i=>i.ras_id==id);
            esr.ext = s;
            Data_context.SubmitChanges();
            this.raschetTableAdapter.Fill(this.pO_DBDataSet.Raschet);
        }

 [Table(Name = "Raschet")]
    class Essense_of_Raschet
    {
        [Column(Name = "Id")]
        public int id1 { get; set; }
 
        [Column(Name = "Shag")]
        public double shag { get; set; }
 
        [Column(Name = "X1")]
        public double x1 { get; set; }
 
        [Column(Name = "X2")]
        public double x2 { get; set; }
 
        [Column(Name = "Extrem")]
        public string ext { get; set; }
 
        [Column(Name = "Ras_Id", IsPrimaryKey = true, IsDbGenerated = true)]
        public int ras_id { get; set; }

Essense_of_Raschet esr = Data_context.Essense_of_Raschet.Where(i=>i.ras_id==id).FirstOrDefault();