Ajax SlideShowExtender slideshow on last slide next button requires two clicks to go back to 1st slide

Ajax SlideShowExtender slideshow on last slide next button requires two clicks to go back to 1st slide, instead it should be one click to 1 slide from last.

<asp:ImageButton runat="Server" ID="prevButton" Text="Prev" CssClass="unit_rot_lef" />
<asp:ImageButton runat="Server" ID="nextButton" Text="Next" CssClass="unit_rot_rit" />
<ajaxToolkit:SlideShowExtender ID="ajxSlideShow" runat="server" TargetControlID="imgProp"
                SlideShowServiceMethod="GetUnitImages" SlideShowServicePath="http://localhost:8090/PSlideService.asmx"
 AutoPlay="false" Loop="true" NextButtonID="nextButton" PreviousButtonID="prevButton" />

any help is this regard is greatly appreciated.

Thx,

H.