Cant add isSameOrAfter plugin: Cannot read property '$i' of undefined

**Describe the bug**

```html

```

```javascript

dayjs.extend(window.isSameOrAfter);

```

```javascript
Uncaught TypeError: Cannot read property '$i' of undefined
at Function.v.extend (dayjs.min.js:1)
at register:724
```

**Expected behavior**
no error

**Information**
- Day.js Version 1.10.4
- OS: [e.g. iOS]
- Browser chrome
- Time zone: [Asia Singapore]

Reproduce the issue in Chrome using the Day.js 1.10.4 and isSameOrAfter CDN script tags shown in the report, then inspect the dayjs.extend(window.isSameOrAfter) call. Confirm the cause of the undefined plugin and verify that the corrected browser setup loads without an error.

Written by the indexing model from the issue text.