Wrong index suggestion when using BinData fields

[isartmontane]

We are running dex on our production dataset but we are not getting the right indexes when a BinData field is used.

For example, for the following log line:

2016-03-14T15:33:49.177+0000 I WRITE    [conn826040] update collection query: { _id: BinData(0, 6165363837656337373438316233343862333033363464663961336231353262) } update: { $set: { a: 1 } } nscanned:1 nscannedObjects:1 nMatched:1 nModified:0 keyUpdates:0 writeConflicts:0 numYields:1 locks:{ Global: { acquireCount: { r: 2, w: 2 } }, Database: { acquireCount: { w: 2 } }, Collection: { acquireCount: { w: 2 } } } 1440ms

We get the following suggestion:

        {
            'queryMask': '{"$query":{"6165363837656337373438316233343862333033363464663961336231353262)":"<val>","_id":"<val>"}}', 
            'namespace': 'db.collection', 
            'recommendation': {
                'index': '{"_id": 1, "6165363837656337373438316233343862333033363464663961336231353262)": 1}', 
                'namespace': 'db.collection', 
                'shellCommand': 'db["collection"].ensureIndex({"_id": 1, "6165363837656337373438316233343862333033363464663961336231353262)": 1}, {"background": true})'
            }, 
            'details': {
                'count': 1, 
                'totalTimeMillis': 1440, 
                'avgTimeMillis': 1440
            }
        },