Error creating control in textbox watermark

i am using ajax watermark on textbox but it show an error "Error creating control in textbox watermark"

code:

<cc1:TextBoxWatermarkExtender ID="TextBoxWatermarkExtender2" runat="server"
                                           TargetControlID="txt_surname0"
                                           WatermarkText="Surname"
                                         WatermarkCssClass="watermark"></cc1:TextBoxWatermarkExtender>