C# - Index Was Out Of Range Error In Added Rows Gridview

I add all row CheckBox in my Gridview helping with this article. Here is my Calculate Button code;

protected void Calculate_Click(object sender, EventArgs e)
{
bool atLeastOneRowDeleted = false;
foreach (GridViewRow row in GridView1.Rows)
{
CheckBox cb = (CheckBox)row.FindControl("ProductSelector");
if (cb != null && cb.Checked)
{
atLeastOneRowDeleted = true;
int productID = Convert.ToInt32(GridView1.DataKeys[row.RowIndex].Value);
Response.Write(string.Format( "This would have deleted ProductID {0}<br />", productID));
}
}
}

But when i do that, getting strange error like this;