CalendarExtender error when clicking out of txtbox

code:

<

asp:TextBoxID="txtStartDate"runat="server"Columns="12"CssClass="text"MaxLength="10"/> <asp:Imagerunat="Server"ID="Image1"ImageUrl="Calendar_scheduleHS.png"/> <asp:RequiredFieldValidatorID="RequiredFieldValidator6"runat="server"ControlToValidate="txtStartDate"CssClass="error"ErrorMessage="Please enter a start date.">*</asp:RequiredFieldValidator><ajaxToolkit:CalendarExtenderID="calendarButtonExtender"runat="server"TargetControlID="txtStartDate"PopupButtonID="Image1"/>