var bulk = db.jstests_bgsec.initializeUnorderedBulkOp();
for( i = 0; i < 100000; ++i ) {         bulk.insert({ i : i });     }
assert.writeOK(bulk.execute());

db.jstests_bgsec.ensureIndex( {i:1} );

> db.jstests_bgsec.getIndexes()
[
	{
		"v" : 1,
		"key" : {
			"_id" : 1
		},
		"name" : "_id_",
		"ns" : "test.jstests_bgsec"
	},
	{
		"v" : 1,
		"key" : {
			"i" : 1
		},
		"name" : "i_1",
		"ns" : "test.jstests_bgsec",
		"background" : true
	}
]

> db.jstests_bgsec.find({i: {$gt: -1}}).hint({i: 1}).explain()
2015-01-15T17:23:21.955-0500 I QUERY    Error: explain failed: {
	"ok" : 0,
	"errmsg" : "error processing query: ns=test.jstests_bgsec limit=0 skip=0\nTree: i $gt -1.0\nSort: {}\nProj: {}\n planner returned error: bad hint",
	"code" : 2
}
    at Error (<anonymous>)
    at Function.throwOrReturn (src/mongo/shell/explainable.js:34:19)
    at constructor.finish (src/mongo/shell/explain_query.js:188:36)
    at DBQuery.explain (src/mongo/shell/query.js:434:25)
    at (shell):1:52 at src/mongo/shell/explainable.js:34


> db.jstests_bgsec.stats()
{
	"ns" : "test.jstests_bgsec",
	"count" : 100000,
	"size" : 4800080,
	"avgObjSize" : 48,
	"numExtents" : 6,
	"storageSize" : 11182080,
	"lastExtentSize" : 8388608,
	"paddingFactor" : 1,
	"paddingFactorNote" : "paddingFactor is unused and unmaintained in 2.8. It remains hard coded to 1.0 for compatibility only.",
	"userFlags" : 1,
	"nindexes" : 1,
	"indexDetails" : {

	},
	"totalIndexSize" : 3262224,
	"indexSizes" : {
		"_id_" : 3262224
	},
	"ok" : 1
}