Call to a member function hasVotedFor() on null

根据知乎系列视频中的对答案点赞功能，做了下例子。是一个vue写的组件。结果出错了Call to a member function hasVotedFor() on null。测试后，发现model中的VoteFor也是同样的错误。去google了类似问题，觉得没有找到答案。不清楚该如何解决此问题。若能得到帮助，不胜感激。

下面是Votecontroller:

 public function users($id)
    {
        $user = Auth::guard('api')->user();

        if($user->hasVotedFor($id)){
            return response()->json(['voted' => true]);
        }

        return response()->json(['voted' => false]);
    }

  public function vote()
    {
        $answer = $this->answer->byId(request('answer'));
        $voted = Auth::guard('api')->user()->voteFor(request('answer'));

        if (count($voted['attached'])>0){
            $answer->increment('votes_count');

            return response()->json(['voted' => true]);
        }

        $answer->decrement('votes_count');

        return response()->json(['voted' => false]);

    }

UserModel:

  public function votes()
    {
        return $this->belongsToMany(Answer::class,'votes')->withTimestamps();
    }

  public function voteFor($answer)
    {
        return $this->votes()->toggle($answer);
    }

  public function hasVotedFor($answer)
    {
        return !! $this->votes()->where('answer_id',$answer)->count();
    }

Route:

Route::post('answer/{id}/votes/users','VoteController@users')->middleware('api');
Route::post('answer/vote','VoteController@vote')->middleware('api');

UserVoteButton.vue:

<template>
    <button
            class="btn btn-default"
            v-bind:class="{'btn-primary': voted}"
            v-text="text"
            v-on:click="vote"
    ></button>
</template>

<script>
    export default {
        props:['answer','count'],
        mounted() {
            this.$http.post('http://localhost:8000/api/answer/' + this.answer +     '/votes/users').then(response =>{
                this.voted = response.data.voted
            })
        },
        data(){
            return {
                voted:false
            }
        },
        computed:{
            text(){
                return this.count
            }
        },
        methods: {
            vote(){
                this.$http.post('http://localhost:8000/api/answer/vote', {'answer':this.answer}).then(response => {
                    this.voted = response.data.voted;
                    response.data.voted ? this.count ++ : this.count --
                })
            }
        }
    }
</script>