Vuex strict mode always leads to "do not mutate vuex store state outside mutation handlers"

[phil294]

Strict mode: https://vuex.vuejs.org/guide/strict.html

When this is enabled, all mutations throw an error (stack trace below), even when commited through a mutation, as you should:

<button @click="$store.commit('setSomeVal')">set someVal</button>

export function createStore() {
	return new Vuex.Store({
		strict: true, // <----------
		state() { return { someVal: false } },
		mutations: {
			setSomeVal(state) {
				state.someVal = true;
			}
		},
		actions: {}, modules: {}
})}

Clicking the button should not generate the error, but it does. I tested this with a clean vue project with nothing else added.

Is this expected behaviour or a bug? If the latter, do you still accept pull requests in case I can track it down?
Thanks!

VM547 chunk-vendors.js:8373 [Vue warn]:
Error in callback for watcher "function () { return this._data.$$state }":
  "Error: [vuex] do not mutate vuex store state outside mutation handlers."

(found in <Root>)
warn @ VM547 chunk-vendors.js:8373
logError @ VM547 chunk-vendors.js:9636
globalHandleError @ VM547 chunk-vendors.js:9631
handleError @ VM547 chunk-vendors.js:9591
run @ VM547 chunk-vendors.js:12317
update @ VM547 chunk-vendors.js:12289
notify @ VM547 chunk-vendors.js:8484
reactiveSetter @ VM547 chunk-vendors.js:8809
reactiveSetter @ vue.runtime.esm.js:1050
setSomeVal @ index.js:16
wrappedMutationHandler @ vuex.esm.js:744
commitIterator @ vuex.esm.js:392
(anonymous) @ vuex.esm.js:391
_withCommit @ vuex.esm.js:526
commit @ vuex.esm.js:390
boundCommit @ vuex.esm.js:335
click @ Home.vue?012f:17
invokeWithErrorHandling @ vue.runtime.esm.js:1854
invoker @ vue.runtime.esm.js:2179
original._wrapper @ vue.runtime.esm.js:6917
VM547 chunk-vendors.js:9640 Error: [vuex] do not mutate vuex store state outside mutation handlers.
    at assert (VM547 chunk-vendors.js:16303)
    at Vue.store._vm.$watch.deep (VM547 chunk-vendors.js:17006)
    at Watcher.run (VM547 chunk-vendors.js:12315)
    at Watcher.update (VM547 chunk-vendors.js:12289)
    at Dep.notify (VM547 chunk-vendors.js:8484)
    at Object.reactiveSetter (VM547 chunk-vendors.js:8809)
    at Object.reactiveSetter [as someVal] (VM547 chunk-vendors.js:8804)
    at Store.setSomeVal (VM548 app.js:1950)
    at wrappedMutationHandler (VM547 chunk-vendors.js:16957)
    at commitIterator (VM547 chunk-vendors.js:16605)

[waydelyle]

try removing vuex-router-sync if you have it.

[phil294]

@waydelyle Nice idea, but not installed. Adding it does not change anything either.
The issue also happens in a fresh vue / ssr-plugin install wihtout any further modifications.