validate() missing 1 required positional argument: 'known_fields'

Using python 3.9 and sqlvalidator==0.0.18

When running `sqlvalidator.parse(query).is_valid()` i'm getting 

```
    def _validate(self):
        self.validated = True
        try:
>           self.errors = self.sql_query.validate()
E           TypeError: validate() missing 1 required positional argument: 'known_fields'
```

The class of self.sql_query is WithStatement