DropDown List Selected Index Change

Question

Helo All,
I am using ASP.NET 3.5. I have a form with a DropDownList contain the User Code. At Selected Index Change the form shows the respecting data. But at times it shows the data but the following error Message is generate.

"Microsoft JScript runtime error: 'this.get_element().style' is null or not an object"

I am using master Page. There is an update panel in the master page.

How can i solve not to generate this error  message when selected index change at DropDownList.

Thanks in advance