Cannot read property 'rows' of null java script error

<script type="text/javascript">
        function Search_Gridview(strKey, strGV) {
            var strData = strKey.value.toLowerCase().split(" ");
            var tblData = document.getElementById(strGV);
            var rowData;
            for (var i = 1; i < tblData.rows.length; i++) {
                rowData = tblData.rows[i].innerHTML;
                var styleDisplay = 'none';
                for (var j = 0; j < strData.length; j++) {
                    if (rowData.toLowerCase().indexOf(strData[j]) >= 0)
                        styleDisplay = '';
                    else {
                        styleDisplay = 'none';
                        break;
                    }
                }
                tblData.rows[i].style.display = styleDisplay;
            }
        }    
    </script>

<div style="border: 1px solid Black; width: 800px; padding: 20px; height: 350px;
            font-size: 20px;">
            Search :
            <asp:TextBox ID="txtSearch" runat="server" Font-Size="20px" onkeyup="Search_Gridview(this, 'gvTest')"></asp:TextBox><br />
            <br />
            <asp:GridView ID="gvTest" runat="server" OnPageIndexChanging="GrdRole_PageIndexChanging" PageSize="10000">
                <columns>
               <asp:TemplateField>
                <ItemTemplate>
                    <asp:CheckBox ID="chkSelect" runat="server" />
                </ItemTemplate>
            </asp:TemplateField>
            </columns>
            </asp:GridView>
        </div>