WatermarkExtender indicator flashes when UpdatePanel starts a timer

I put UpdatePanelwith Timeras a trigger on my web page. Also, outside the panel I put TextBoxwith WatermarkExtender.

When the timer goes off, it is updated UpdatePanel. In addition, the watermark blinks (it disappears from the text box and reappears). And this happens every 1 second. This is a big problem.

Any help please?