uni.chooseMedia报错

<template>  
    <view>  
        <button @click="chooseMig">选择图片</button>  
    </view>  
</template>  

<script>  
    export default {  
        data() {  
            return {  

            }  
        },  
        onLoad() {  

        },  
        methods: {  
            chooseMig :function(){  
                uni.chooseMedia({  
                  count: 3,  
                  mediaType: ['image','video'],  
                  sourceType: ['album', 'camera'],  
                  maxDuration: 30,  
                  camera: 'back',  
                  success(res) {  
                    console.log(res.tempFiles)  
                  }  
                });  
            }  
        }  
    }  
</script>  

<style>  

</style>  

点击按钮后，报错，提示：
https://github.com/vuejs/vue-devtools
chunk-vendors.js:2765 [Vue warn]: Error in v-on handler: "TypeError: uni.chooseMedia is not a function"

found in

---> at pages/index/index.vue

• 请问这是怎么回事呀？