a4j:commandButton not executing A4J.AJAX.onError()

[jingjing_girl]

my pages has a a4j:commandButton,when the action method execut,it appear excetion,this page will show
Request error,status:500 Internal Server Error
Message:undefined

i want to deal it ,so in my page, use A4J.AJAX.onError()

A4J.AJAX.onError = function(req,status,message) {
alert('error');
};

A4J.AJAX.onExpired = function(loc,expiredMsg){
alert('error2');
} ;


but it not execut, still show
Request error,status:500 Internal Server Error
Message:undefined


why? who can tell me ??

thanks very much!!