dropdownlist empty on button click

HI Friends,

On button_click(postback), my dropdownlist of gridview is getting blank, so i m getting error of "Object Reference...." on the line "ddl.selecteditem.value"

Also, dropdownlists selectedindexchanged event is fired on button click(Page Postback), which is making dropdownlist to go blank. AutoPostBack of dropdownlist is set as False, still the event is fired

Pls help