mocha + shouldjs Object Assertion没有方法'indexOf'

Question

我正在使用mochajs +应该承诺作为测试框架。

我正在尝试测试蓝鸟的承诺，我得到了

Unhandled rejection TypeError: Object #<Assertion> has no method 'indexOf'
    at Assertion.<anonymous> ([app path]\node_modules\sh
ould\lib\ext\string.js:22:32)

测试：

obj.certainMethod(opts).should.be.eventually.an.Array();

如果我手动运行该方法，我会得到一个像这样的JSON：

[
  {
    "_id": "55ce4fc18a051bc4a1aed7f5",
    "index": 0,
    "guid": "61d6d6a9-5864-4c48-8377-04e9f4260112",
    "isActive": true,
    "balance": "$3,763.34",
    "picture": "http://placehold.it/32x32",
    "age": 30,
    "range": [1,2,3]
  },
  {
    "_id": "55ce4fc1a5b9b94d9d3888f5",
    "index": 1,
    "guid": "08f6ce47-997d-453d-a16a-030a28726191",
    "isActive": false,
    "balance": "$1,805.68",
    "picture": "http://placehold.it/32x32",
    "age": 31,
    "range": [1,2,3]      
  }
]

关于什么是错的任何想法？