update cache in mutation

[DG3D]

Hi,

I'm unable to update the cache after my mutation and I don't understand how I can manage this.

I followed the documentation here : Making all other cache updates

I found this issue that seems to be a bit ignored...

Here are the 2 ways I tried :

    const { mutate: addNewUser } = useMutation(insertUser, () => ({
      variables: {
        firstname: firstname.value,
      },
      update: (cache, { data: { addUser } }) => {
        const data = cache.readQuery({ query: getUsers })
        data.users.push(addNewUser)
        cache.writeQuery({ query: getUsers, data })
      }
    }));

    const { mutate: addNewUser } = useMutation(insertUser, () => ({
      variables: {
        firstname: firstname.value,
      },
      update: (cache, { data: { addUser } }) => {
        const data = cache.readQuery({ query: getUsers })
        console.log(data.users)
        data.users = [...data.users, addNewUser]
        cache.writeQuery({ query: getUsers, data })
      }
    }));

And both throws this error :

Uncaught (in promise) Error: Cannot add property 39, object is not extensible
    at new ApolloError (index.js?6e99:29)
    at Object.error (QueryManager.js?1d9d:137)
    at notifySubscription (module.js?f89c:137)
    at onNotify (module.js?f89c:176)
    at SubscriptionObserver.error (module.js?f89c:229)
    at eval (asyncMap.js?a45e:27)

Does anybody can help me with this issue ? Maybe giving me a workaround ?