Checkbox autopostback does not work when i use Ajaxtoolkit TextBoxWatermarkExtender

Checkbox autopostback does not work when i use Ajaxtoolkit TextBoxWatermarkExtender,

I have textbox and TextBoxWatermarkExtender 

and checkbox with autopostback=tre

when i check the checkbox i disable the button1

but it works if i removed TextBoxWatermarkExtender else the auotpostback does not work .

Thansk