我使用的是mongodb 2.6.1。但是，我无法使用sparse创建唯一索引。目前，我有以下索引：

> db.products.getIndexes()
[
    {
        "v" : 1,
        "key" : {
            "_id" : 1
        },
        "name" : "_id_",
        "ns" : "snapyshop_production.products"
    },
    {
        "v" : 1,
        "key" : {
            "pickup_location" : "2dsphere"
        },
        "name" : "pickup_location_2dsphere",
        "background" : true,
        "ns" : "snapyshop_production.products",
        "2dsphereIndexVersion" : 2
    },
    {
        "v" : 1,
        "key" : {
            "category_id" : 1
        },
        "name" : "category_id_1",
        "background" : true,
        "ns" : "snapyshop_production.products"
    },
    {
        "v" : 1,
        "key" : {
            "_keywords" : 1
        },
        "name" : "_keywords_1",
        "background" : true,
        "ns" : "snapyshop_production.products"
    }
]

但是当我运行这个命令时，它会打印出错误：

> db.products.ensureIndex( { source_url: 1 }, { background: true, sparse: true, unique: true } )
{
    "createdCollectionAutomatically" : false,
    "numIndexesBefore" : 4,
    "ok" : 0,
    "errmsg" : "E11000 duplicate key error index: snapyshop_production.products.$source_url_1  dup key: { : null }",
    "code" : 11000
}

我真的不知道该怎么修。