Why does Haystack rest API return empty string answer with highest score ?

[Petitbonzai31]

I have a haystack rest API deployed on an internal server.
It works fine but sometimes when a post a query i get this kind of response :

"query": "comment dépanner une panne internet", "answers": [ { "answer": "", "type": "extractive", "score": 0.4872324064557315, "offsets_in_document": [ { "start": 0, "end": 0 } ], "offsets_in_context": [ { "start": 0, "end": 0 } ], "meta": {} }, { "answer": "Désactivez votre firewall temporairement et reconnectez-vous à internet..", "type": "extractive", "score": 0.3117782771587372, "context": "Désactivez votre...

The first answer is an empty string with a score of 0.48 !
The second answer is right but the score is lower (0.31).

How can i avoid empty string answer ?
Which params can be sent into query request ? i could not find anything about it in Haystack docs ?

Thanks