serialize is not a function

**What version of Ajv are you using? Does the issue happen if you use the latest version?**
6.12.5

**Ajv options object**

```javascript

meta: {
$schema: 'http://json-schema.org/draft-07/schema#',
},
$data: true,
allErrors: true,
jsonPointers: true,
}

```

**JSON Schema**

```json

```

**Sample data**

```json

```

**Your code**

```javascript
const defaultOptions = {
meta: {
$schema: 'http://json-schema.org/draft-07/schema#',
},
$data: true,
allErrors: true,
jsonPointers: true,
};
const ajv = new Ajv({
...defaultOptions,
});
```

**Validation result, data AFTER validation, error messages**

```

```

**What results did you expect?**

expect to init an ajv object

**Are you going to resolve the issue?**
throw error "serialize is not a function"
I set a debug point, it seems to be a string, not a function