jquery cookie报错：has no method 'cookie'

$(document).ready(function () {
    var a = $.cookie("myAddress");
}
报如下错误：
Uncaught TypeError: Object function ( selector, context ) {
// The jQuery object is actually just the init constructor 'enhanced'
return new jQuery.fn.init( selector, context, rootjQuery );
    } has no method 'cookie'
 
而如果这样写是可以：
var a = $.cookie("myAddress"); 
$(document).ready(function () {
    ...
}
jquery.cookie插件先已经引入了，为什么放在read()里面就不行了呢