C# - FCK Editor + Update Panel + ValidateRequest="false" ?

When I set fckEditor to Some Value like

fckDescription.Value = "Description Text";

It creates problems such as Update Panel not doing Async PostBack for DropDownList control and gives error when DropDownList selection changes:Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occurred while processing the request on the server. The status code returned from the server was: 500

But when I Comment out these lines
// fckDescription.Value = "Description Text";
It Works Fine... I am wondering why it is so !!?!!

Also, fckEditor is outside UpdatePanel and DropDownList Control is inside UpdatePanel.