Module build failed: TypeError: this._init is not a function

vue-strap is a  vuecomponent , and  i want to use it like this:

require('vue-strap').alert

but it report errors below:

Module build failed: TypeError: this._init is not a function
    at Object.Vue (E:\vue\vue-strap1\node_modules\vue-strap\node_modules\vue\src
\vue.js:19:8)

the code  in the vue-strap/src/main.js  file:

var _AlertVue = require('./Alert.vue');

exports.alert = _AlertVue;
