Cannot update states using this in Pinia actions

[col-bc]

I'm very new to Pinia and had only a little experience with Vuex. I'm trying to update the state from inside an action function but keep getting this error:

TypeError: Cannot set properties of undefined (setting 'token')
    at Proxy.setToken

This is my @/store/index.js fils

import { defineStore } from 'pinia'
import axios from 'axios'

export const useStore = defineStore('main', {
  state: () => ({
    user: {},
    token: '',
    isLoggedIn: false,
    lastFetch: new Date(1970, 0, 1),
  }),
  getters: {
    isLoggedIn: state => state.token !== '',
    getUserData: state => state.user,
    getToken: state => state.token,
    isDataStale: state => new Date() - state.lastFetch > 60000,
    bearerToken: state => `Bearer ${state.token}`
  },
  actions: {
    setUserData: (payload) => {
      this.user = payload  // <-- error occurs here
    },
    fetchUserData: async (force = false) => {
      if (this.isDataStale || force) {
        const response = await axios.get("http://localhost:5000/api/user/", {
          headers: {
            Authorization: `Bearer  ${this.token}`,
          },
        })
        this.setUserData(response.data);
        this.lastFetch = new Date();
      }
    },
    setToken: (payload) => {
      this.token = payload
    },
    logout: () => {
      this.token = ''
      this.user = {}
      this.lastFetch = new Date(1970, 0, 1)
    }
  },

  persist: true,
})

The only way I've been able to successfully update state is if I use the this.$patch() function from within a component or view.

Thank you in advance!

[col-bc]

I believe I found the problem. If use the following syntax for defining actions, they execute without error:

actions: {
  setToken(token) {
     this.token = token
  },
}

[lex-gh]

Arrow functions don't have their own bindings to this
MDN