Popupextender not working onfocus

<asp:TextBox ID="LicenseBox" runat="server"  Width="140px" OnTextChanged="LicensePlate_Changed"
          AutoPostBack="true" CssClass="UpperCase" TabIndex="1"
              onfocus="javascript:this.style.background='#00FF00';"
              onblur="javascript:this.style.background='#FFFFFF';"></asp:TextBox><asp:BalloonPopupExtender ID="BalloonPopupExtender1" runat="server"
          TargetControlID="LicenseBox"
          BalloonPopupControlID="ButtonAdvise"
          Position="BottomRight"
          BalloonStyle="Cloud"
          BalloonSize="Small"
          CustomCssUrl="CustomStyle/BalloonPopupOvalStyle.css"
          CustomClassName="oval"
          UseShadow="true"
          ScrollBars="Auto"
          DisplayOnMouseOver="true"
          DisplayOnFocus="true"
          DisplayOnClick="true"></asp:BalloonPopupExtender>

This is my code for the extender, but onfocus does not work. Mouseover and Click work fine. The textbox changes color so it has focus.

Any ideas would be appreciated.

Ross