ASP.NET TableHeaderRow controller produces tr not th

This is my code

<asp:Table runat="server" ID="table" CssClass="table">
            <asp:TableHeaderRow>
                <asp:TableCell>Date</asp:TableCell>
                <asp:TableCell>TotalInboundCalls</asp:TableCell>
                <asp:TableCell>GhostCalls</asp:TableCell>
                <asp:TableCell>AnserdCalls</asp:TableCell>
                <asp:TableCell>AVGDurationTime</asp:TableCell>
                <asp:TableCell>AVGTakingTime</asp:TableCell>
                <asp:TableCell>AVGWaitingTime</asp:TableCell>
                <asp:TableCell>AvgRingingTime</asp:TableCell>
                <asp:TableCell>AvgHoldTime</asp:TableCell>
            </asp:TableHeaderRow>
        </asp:Table>

      

        
        
        
      

    

The generated html tr

not th

, what did I do wrong?