Conditional Display of Download Button in ASP.NET GridView TemplateField

<asp:TemplateField HeaderText="Download Document">
    <ItemTemplate>
        <asp:ImageButton ID="btnDownload" runat="server"
         CommandName="Download"
         CommandArgument='<%# Eval("Document_ID") & "," & Eval("Document_Name") %>'
         ImageUrl="download.png"
         ToolTip='<%#"Download " & Eval("Document_Name") %>'
         Visible='<%# Not(Eval("Document_ID") = -1) %>' />
    </ItemTemplate>
</asp:TemplateField>