Error in inserting record

protected void btnAdd_Click(object sender, EventArgs e)
    {
 
        SqlConnection conNwind;
        SqlCommand cmdSelect, cmdSelectID;
        SqlDataReader dtrCustomer;
 
 
        string connStr = ConfigurationManager.ConnectionStrings["ConnectionNWind"].ConnectionString;
        conNwind = new SqlConnection(connStr);
 
 
        conNwind.Open();
 
        string select = "Select * from Customers where CustomerID=@ID";
        cmdSelectID = new SqlCommand(select, conNwind);
 
        cmdSelectID.Parameters.AddWithValue("@ID", txtId.Text);
        dtrCustomer = cmdSelectID.ExecuteReader();
 
 
        if (dtrCustomer.Read())
        {
            Label1.Text = "ID already exist!";
 
        }
 
        else
        {
            string strInsert = "Insert into Customers(CustomerID,Companyname,Contactname,Phone) values(@customerId,@companyname,@contactname,@phone)";
 
            cmdSelect = new SqlCommand(strInsert, conNwind);
 
            cmdSelect.Parameters.AddWithValue("@customerId", txtId.Text);
            cmdSelect.Parameters.AddWithValue("@companyname", txtCompanyName.Text);
            cmdSelect.Parameters.AddWithValue("@contactname", txtContactName.Text);
            cmdSelect.Parameters.AddWithValue("@phone", txtPhone.Text);
 
            int intAdd = cmdSelect.ExecuteNonQuery();
 
            if (intAdd != 0)
                Label1.Text = "Record successfully added into database!";
 
            else
                Label1.Text = "Record not added into database!";
        }
 
        dtrCustomer.Close();
        conNwind.Close();
 
    }