Target="_blank" doesn't work in IE8

<asp:TemplateField HeaderText="User Name" SortExpression="UserName"> <ItemTemplate> <asp:HyperLink ID="UserName" runat="server" Target="_blank" NavigateURL='<%# FormatURL(Eval("UserName").ToString()) %>' Text='<%# Eval("[UserName]") %>' ></asp:HyperLink> </ItemTemplate> </asp:TemplateField>
When I try to open a new window when hyperlink is clicked. It doesn't do anything in IE 8, works in IE 7 and firefox.
I have tried window.open() but doesn't work. Any ideas ?
Thankyou
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