Vue - how to access the state within MutationAction

I can't access the state inside @MutationAction

I'm using:

Nuxt.js v2.13.3 "vuex-module-decorators": "^0.17.0"

import { Module, VuexModule, MutationAction } from 'vuex-module-decorators'

@Module({
  name: 'counter',
  stateFactory: true,
  namespaced: true
})
export default class Auth extends VuexModule {
  public counter: number = 5

  @MutationAction({ mutate: ['counter'] })
  async set() {
    console.log(this.counter)
    return { counter }
  }
}

output: undefined


this way it works

@MutationAction({ mutate: ['counter'] })
async set() {
  console.log((this.state as any).counter)
  return { counter }
}

output: 5


but if I set a value before reading it, everything goes well

@MutationAction({ mutate: ['counter'] })
async set() {
  this.counter = '20'
  console.log(this.counter)
  return { counter }
}

output: 20

can someone help me make this work? this.counter