I have a code which is selecting the checkbox and working very well in the internet explore 7. But could not selecting the checkbox in Internet Explorer 6.0

Can any one tell me why this is not working in the IE 6.0. Here is source code

<

telerik:RadComboBox ID="combobox1" runat="server" HighlightTemplatedItems="true"

Height="120px" Skin="Default" DataTextField="CityName" DataValueField="CityNameid"

Text="City" AutoPostBack="true">

<CollapseAnimation Duration="300" Type="OutQuint" />

<HeaderTemplate>

<asp:Label ID="lblCity" Height="20px" Text="City" runat="server"></asp:Label>

</HeaderTemplate>

<ItemTemplate>

<asp:CheckBox AutoPostBack="true" runat="server" ID="chkbxCity" OnCheckedChanged="getSelectedCity" /> <%#DataBinder.Eval(Container.DataItem, "CityName")%> 
</ItemTemplate>

<CollapseAnimation Duration="300" Type="OutQuint" />

</telerik:RadComboBox>

I have checked in debug mode when I ran it in the IE 7 it goes to the function 
OnCheckedChanged="getSelectedCity"

but when I ran it in the IE 6 It does not go in the above stated function

any help

Feejaz

protected void OnSelectedIndexChanged_ddUserSearchContext(object sender, EventArgs e)

{

RadComboBox MakerRadCombo = (RadComboBox)RadToolbar1.FindControl("MakerRadCombo");

DropDownList ddUserChoce = (DropDownList)RadToolbar1.FindControl("ddUserSearchContext");

TextBox tbSearchContractNo = (TextBox) RadToolbar1.FindControl("tbSearchContractNo");

if (ddUserChoce.SelectedValue == "Contract")

{

MakerRadCombo.Visible =

false;

tbSearchContractNo.Visible =

true;

}

else if (ddUserChoce.SelectedValue == "Maker")

{

MakerRadCombo.Visible =

true;

tbSearchContractNo.Visible =

false; ;

}

}

It fails because tbSearchContractNo  = null (it's invisible in the beginning)

Any insight would be helpful.

Thanks in advance,
Al Gorup

<telerik:RadToolTipManager runat="server" ID="RadTipPW" ShowEvent="OnFocus" Skin="Default2006" ShowDelay="0" AutoCloseDelay="4000" Font-Size="Smaller" Width="150px" Position="MiddleRight" RelativeTo="Element">

<TargetControls>

<telerik:ToolTipTargetControl TargetControlID="txtPassword" IsClientID="False" />

</TargetControls>

</telerik:RadToolTipManager>