Wrong index is being used, causing inefficient queries

I have a collection with two fields. One is a string and another is an ObjectId.
I have indices on both fields.
When doing a query such as {'string_field' : 'some value', 'other_obj_id' : {$exists : true}}, the index used for the query is consistently the one on 'other_obj_id' and not the one on 'string_field', although that one would be much faster.

This staretd to happen as soon as I've upgraded to 2.6.4 and is a degradation from 2.6.1 !