MultiHandleSliderExtender could not be set on property MultiHandleSlid

I was working with the AJAX MultiHandleSliderExtender control, the thing that bother me the most is the error listed below at design time. The code didn't throw any error at runtime. Error Creating Control - MultiHandleSliderExtender1 'cc1:MultiHandleSliderExtender could not be set on property MultiHandleSliderTargets'