There is no property with the name 'displayState'

1

Please try to help me. CS4 shows there is an error “There is no property with the name ‘displayState’.”

Source:
on (rollOver){
this.gotoAndPlay(“s1”);
}
on (rollOut){
this.gotoAndPlay(“s2”);
}
on (release){
if (Stage.displayState == “normal”){
Stage.displayState = “fullScreen”;
} else {
Stage.displayState = “normal”;
} // end else if
}

Thank you for any help…