Flash RollOver Error #1119

[Photo_Susie]

I'm pretty new to flash & I'm trying to complete a simple roll over action & I keep getting this error message.

1119: Access of possibly undefined property onRollOver through a reference with static type flash.display:SimpleButton.

[Photo_Susie]

this is what I'm trying to do:

thumb1.onRollOver=function () {
this._alpha=100;
}
thumb1.onRollOut=function() {
this._alpha=50;
}