Uncaught TypeError is not a function - Laravel Vue

Inside the template of the Event.vue component I have a select that reads from the wallet table, it is a normal select to choose only one type of wallet (it is not multiselect), for that select I use the vue-multiselect component to make a simple select, the code is the following

<template>
  <div class="form-group row">
    <label for="" class="col-2 col-form-label">Bitellera</label>
    <div class="col-3">
        <multiselect name="wallet" v-model="walletvalue" 
                                 track-by="id" 
                                 label="name" 
                                 placeholder="Selecciona" 
                                 :options="wallets" 
                                 :searchable="false"
                                 :allow-empty="false">
        </multiselect>
     </div>
</template>
 <script>
    export default {  
      components: { 
          Multiselect,
          Datepicker,
      },
      props: ['auth'],     
      data () {
          return {
              wallets:        [],
              walletvalue:    [],
          }
      },
      created() {
        this.getWallet();
      },
      methods: {
         getWallet(){
            let urlWallet = '/dashboard/wallets';
            axios.get(urlWallet)
                .then((response) => {
                    this.wallets = response.data;
                })
                .catch((err) => {

                })
         },
         createdEvent(){
            let urlEvent = '/dashboard/addevent';
            let idwallet = this.walletvalue.map(e=> e.id);
            const eventData = {
                'wallet'        : idwallet,
            }
                axios.post(urlEvent, eventData)
                .then((response) => {
                    console.log(ok);
                })
                .catch((err) => {

                })
        }
  }
 </script>

As Vue brings the complete object, I only need to show the id of the selected wallet, for that I use the js map function, the problem is that it throws me the following error when making the view work

Uncaught TypeError: this.walletvalue.map is not a function
at VueComponent.createdEvent (app.js:79132)
at submit (app.js:79192)
at invoker (app.js:22317)
at HTMLFormElement.fn._withTask.fn._withTask (app.js:22116)

How can i fix this? I use Laravel 5.6 and Vuejs 2