Here is my example code:
```javascript
    test('commits setContributionId', (done) => {
      const state = {
        comments: []
      }
      testAction(action, 42, state, [
        // ...
        { type: 'isLoading', payload: false}
      ], done)
    })
```
I'm passing `42` as an argument to `testAction` and I want to verify that `commit('isLoading', payload)` is executed. Here the payload is not the payload of the testAction helper (in my case 42) but the second argument of `commit`. That might be `false` and my case it is. If I change the payload of the last expected commit to be `true` no assertion error is raised. The fix is to make the if condition in the mocked commit method more specific.

If a user specifies an expected payload, the assertion **must** be
executed.

Let's say your store action looks like this:

```
const somePayload = undefined
// this is the bug to be exposed, `somePayload` should not be undefined

commit('isLoading', somePayload)
```

And the developer defines an expected payload in the test:

```js
testAction(action, 42, state, [
  //...
  { type: 'isLoading', payload: false }
], done)
```

The test will unexpectedly pass as the assertion is skipped.
Instead we always want to check the assertion if any payload was
defined by the developer.

Read: vuejs#1425 (comment)

Here you go