Error when saving DropDownList value to Database: error converting data type nvarchar to int

I am using  a  viewstate to store the company Ids and splitting at hyphen.

  protected void onCompanychange(object sender, EventArgs e)
        {
                ViewState["companyID"] += companydropdown.SelectedValue + "-";
                String companyids = ViewState["companyID"].ToString();
                string pattern = "-";
                company_array = Regex.Split(companyids, pattern);
                company_length = company_array.Length;            
        }
      

 company array is a string array  where i have stored the company ids

the problem is company_array[0] has the initial value of the dropdownlist(company) --select company which is why m getting the error(cannot convert data type nvarchar to int) .  I have used the same logic in many places in my project it did not give any error.

Please provide me some solutions

int count = 0;
                    company_length = company_length - 1;
                    while (count < GridView3.Rows.Count && compid_counter < company_length )
                    {                      
                        cmd = new SqlCommand("insertWorkDetais", conn);
                        cmd.CommandType = CommandType.StoredProcedure;
                        cmd.Parameters.AddWithValue("@trainerId",trainerID);
                        cmd.Parameters.AddWithValue("@company_Id", company_array[compid_counter]);
                        cmd.Parameters.AddWithValue("@joined_date", Convert.ToDateTime(GridView3.Rows[count].Cells[1].Text));
                        cmd.Parameters.AddWithValue("@left_date", Convert.ToDateTime(GridView3.Rows[count].Cells[2].Text));
                        cmd.ExecuteNonQuery();
                        count++;
                        compid_counter++;
                    }