I have a collection, the data structure:

{ "_id" :

{ "h" : NumberLong("331673610000000"), "s" : ISODate("2011-01-10T05:05:00Z") }

, "intervals" : [ ] }

There is an index on _id.s:
mongos> db.readings.getIndexes()
[
{
"v" : 1,
"key" :

{ "_id" : 1 }

,
"ns" : "fivemin.readings",
"name" : "id"
},
{
"v" : 1,
"key" :

{ "_id.s" : 1 }

,
"ns" : "fivemin.readings",
"name" : "_id.s_1",
"background" : true
}
]

in the shell I am able to query for information using this index:
mongos> db.readings.find(

{"_id.s": ISODate("2011-01-10T05:05:00Z")}

).count()
13135

With the ruby driver my queries return an empty set:

db['readings'].find("_id.s" =>

{"$gte" => start_date, "$lte" => start_date}

).count()
=> 0

Is this a defect? Should I be writing the ruby query differently?