The 'SelectedIndex' and 'SelectedValue' attributes are mutually exclusive C#

I am getting SelectedIndex and SelectedValue attributes are mutually exclusive error. I restored the previous working code but still getting the same error. I don't believe I made any database related change.
The code failes at the below section where it attempts to pass the Incident number to PUPDATE.ASPX page. The PUPDATE.ASPX page pull detailed record for the specific Incident number. Any guidance or direction on this is greatly appreciated
protected void gvIncidents_SelectedIndexChanging(object sender, GridViewSelectEventArgs e) { Session["SN_PDT_SELECTED_INCIDENT"] = gvIncidents.DataKeys[e.NewSelectedIndex].Value; Response.Redirect("PUPDATE.aspx"); }