TypeError: slice indices must be integers or None or have an __index__ method

Hi,
I've been making attempts to use RAGatouille for document retrievals. I'm currently testing it on a junk of 30k emails. After indexing a few of the documents I would want to use the ```add_to_index``` function to add new documents to the index. I'm adding 1000 new documents in a loop using ```add_to_index``` but it fails to do so after 21 iterations or after indexing 20000 documents and throws:

```
TypeError: slice indices must be integers or None or have an __index__ method
```

I've written all this code in this saved Kaggle [notebook](https://www.kaggle.com/code/kianindeed/ragatouille-testing/notebook). I would request you to check it out.

I've tested this multiple times for different numbers and lengths of chunks and seems to give the same ```TypeError``` after indexing a particular amount of document chunks.

Start by running the linked Kaggle notebook and reproducing the failure in the loop calling add_to_index after roughly 20,000 document chunks. Trace the TypeError from that call and verify that indexing the 30,000-email workload and adding later batches completes without the slice-index error.

Written by the indexing model from the issue text.