vue2.0动态绑定图片src属性值初始化时报错

<template>
    <div>
        <img :src="imgUrl">
    </div>
</template>

<script>
export default {
    data(){
        return {
            captcha_id: "" 
        }
    },
    computed: {
        imgUrl(){
            return ‘ http://www.demo.com/static/ ‘+ this.captcha_id + ‘.png‘
        },
    },
    methods: {
        init(){
                // 此处省略一个请求 ，假设成功返回数据为 res
                this.captcha_id = res.data.captcha_id;
        },
    }   
    created(){
        this.init();
    }
}

</script>

<style lang="scss" scoped>

</style>    

computed: {
        imgUrl(){
            if(this.captcha_id){
                return this.$store.state.cmnUrl +‘/v1/cmn/captcha/new/‘ + this.captcha_id + ‘.png‘
            }else{
                return null;
            }
        },
    },