帮我看看这段代码为什么出错了？

function file_open(event)
{
var nsIFilePicker = Components.interfaces.nsIFilePicker;
var fp = Components.classes["@mozilla.org/filepicker;1"].createInstance(nsIFilePicker);
fp.init(window, "Select a File", nsIFilePicker.modeOpen);
}

错误： [Exception... "'Ö^' UnnamedClass.classes öCP
³' when calling method: [nsIDOMEventListener::handleEvent]"  nsresult: "0x8057001e (NS_ERROR_XPC_JS_THREW_STRING)"  location: "<unknown>"  data: no]