Has been fixed in v1 indexes. Mostly creating a ticket for this so we can track the issue if reported by users.

Here is the problem for v0 indexes:

> c.drop()
true
> c.ensureIndex(

{a:1}

,

{v:0}

)
> c.save(

{a:[]}

)
> c.find(

{a:null}

) // key matching only

{ "_id" : ObjectId("4ee8f8d00cf334c01d36adc3"), "a" : [ ] }

> c.find(

{a:null,b:null}

) // match full doc - [] not matching null
> c.count(

{a:null}

) // fast count bypasses matcher
1
> c.save(

{a:[1,2]}

) // make the index multikey
> c.find(

{a:null}

) // key matching does not occur for multikey indexes, so match full doc
> c.find(

{a:null,b:null}

) // match full doc
> c.count(

{a:null}

) // fast count bypasses matcher
1

We've fixed it for v1 indexes:

> c.drop()
true
> c.ensureIndex(

{a:1}

)
> c.save(

{a:[]}

)
> c.find(

{a:null}

)
> c.find(

{a:null,b:null}

)
> c.count(

{a:null}

)
0
> c.save(

{a:[1,2]}

)
> c.find(

{a:null}

)
> c.find(

{a:null,b:null}

)
> c.count(

{a:null}

)
0
>