Allowed to build {<field>:0} index

it is allowed to create an

{a:0}

index. This index has unexpected behavior with sorting

> db.some.insert(

{a:1}

)
> db.some.ensureIndex(

{a:0}

)
> db.some.find().sort(

{a:1}

)

{ "_id" : ObjectId("51254055e6f520cff750a549"), "a" : 1 }

> db.some.find().sort(

{a:1}

).explain()
{
"cursor" : "BtreeCursor a_0 reverse",
"isMultiKey" : false,
"n" : 1,
"nscannedObjects" : 1,
"nscanned" : 1,
"nscannedObjectsAllPlans" : 1,
"nscannedAllPlans" : 1,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"a" : [
[

{ "$maxElement" : 1 }

,

{ "$minElement" : 1 }

]
]
},
"server" : "logab-osx.local:27017"
}
> db.some.find().sort(

{a:-1}

).explain()
{
"cursor" : "BtreeCursor a_0 reverse",
"isMultiKey" : false,
"n" : 1,
"nscannedObjects" : 1,
"nscanned" : 1,
"nscannedObjectsAllPlans" : 1,
"nscannedAllPlans" : 1,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"a" : [
[

{ "$maxElement" : 1 }

,

{ "$minElement" : 1 }

]
]
},
"server" : "logab-osx.local:27017"
}
>