I have a primary collection users:

{ _id: ObjectId, follow: [ObjectId], primary: ObjectId }

primary field is optional in document
I have a non-sparse index:

{ follow: 1, primary: 1 }

When I try to query all followers without primary:
db.users.count(

{ follow: ObjectId, primary: null }

)
it uses only follow field, fetches all these documents and then filters out documents where primary is null.

Here is a result from explain:
"winningPlan" : {
"stage" : "COUNT",
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"primary" :

{ "$eq" : null }

},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" :

{ "follow" : NumberInt(1), "primary" : NumberInt(1) }

,
"indexName" : "follow_1_primary_1",
"isMultiKey" : true,
"direction" : "forward",
"indexBounds" :

{ "follow" : [ "[ObjectId('5382f987832983fb3a5ae9d4'), ObjectId('5382f987832983fb3a5ae9d4')]" ], "primary" : [ "[null, null]" ] }

}
}
}

It shows 'filter' stage that absolutely not needed.
It drastically drops down a performance of an application.

I checked that if I put primary as 0 - it works fine.
The problem is query with null value.