Vue.js + vue-resource + vue-router = Lexical declaration error?

When trying to use Vue.js with vue-resource and vue-router a error is shown:

ReferenceError: can't access lexical declaration `vm' before initialization

As far a I tracked the error down it only appears when the page is loaded with an initial route, e.g. test.com/#/files (error) and test.com/ (no error). If all code of requestDirectory() is replaced by next() the error will not be shown. So it seems inherit to code structure. Any ideas?

const routerComponents = {
    home: {     template: '<p>Home ...</p>' },
    files: { template: '<p>Files</p>' }
};

function requestDirectory(to, from, next) {
    console.log("Loading files in ...");
    vm.$http.post('/api/dir', {dir: 'photos'}).then((res) => {
        console.log("done");
        next()
    }, (err) => {});
}

const router = new VueRouter({
    routes: [
        { path: '/', component: routerComponents.home },
        { path: '/files', component: routerComponents.files, beforeEnter: requestDirectory }
    ]
});

const vm = new Vue({
    data: {
        title: 'Home'
    },
    router: router
});

window.onload = function() {
    vm.$mount('#app');
};