/filter returns Unexpected error for string columns
For example:

https://datasets-server.huggingface.co/filter?dataset=tapaco&config=af&split=train&where=language=%27af%27

```
{"error":"Unexpected error."}
```

/filter returns Unexpected error for string columns: For example:

https://datasets-server.huggingface.co/filter?dataset=tapaco&config=af&split=train&where=language=%27af%27

```
{"error":"Unexpected error."}
```
