Impossible to find with index

[pedefe]

Hello,
I build a simple collection with several fields like "site" (string), "zone" (string), class" (string), "timeStamp" (int), "count" (int).
I have build indexes with all combinations of fields : site, site+zone, size+class....until site+zone+class+timeStamp
Depending of my search criteria (for exemple on site and timestamp), I call .find method with the corresponding index (i.e. site+timeStamp).
So I use it with "use_index" option : name_of_index.
Error is always : "pouchdb Could not find that index or could not use that index for the query"
Any idea ?
Best regards.