MongoDB 索引删除失败

Question

数据库 mongodb v3.0.6
通过 db.test.getIndexes()得到索引如下：

{
     "v" : NumberInt(1),
     "key" : {
         "startTime" : NumberInt(1),
         "endTime" : NumberInt(1),
         "status" : NumberInt(1)
     },
     "name" : "startTime_1_endTime_1_status_1",
     "ns" : "test",
     "background" : true
}

尝试使用以下三种写法删除索引均告知失败：

db.test.dropIndex('startTime_1_endTime_1_status_1')

db.test.dropIndex({startTime:1,endTime:1,status:1})

db.test.dropIndex({startTime:NumberInt(1),endTime:NumberInt(1),status:NumberInt(1)})

错误结果分别对应如下：

{
    "message" : "index not found with name [startTime_1_endTime_1_status_1]",
    "nIndexesWas" : 10,
    "ok" : 0,
    "name" : "MongoError"
}

{
    "message" : "can't find index with key:{ startTime: 1.0, endTime: 1.0, status: 1.0 }",
    "nIndexesWas" : 10,
    "ok" : 0,
    "name" : "MongoError"
}

{
    "message" : "can't find index with key:{ startTime: 1, endTime: 1, status: 1 }",
    "nIndexesWas" : 10,
    "ok" : 0,
    "name" : "MongoError"
}

有没有大神解答下，这是什么情况，以及要怎么处理才可以？