egg: egg-socket.io start error “Cannot convert undefined or null to object”

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我按照egg－socket.io项目中README的步骤，用npm并配置了这个插件，但是在npm run dev的时候一直报如下错误：

/root/projects/agent-server/node_modules/_egg@1.3.0@egg/lib/application.js:38
      throw e;
      ^

TypeError: Cannot convert undefined or null to object
    at Namespace.use (/root/projects/agent-server/node_modules/_socket.io@2.0.2@socket.io/lib/namespace.js:103:21)
    at initNsp (/root/projects/agent-server/node_modules/_egg-socket.io@2.0.0@egg-socket.io/lib/io.js:80:9)
    at module.exports.app (/root/projects/agent-server/node_modules/_egg-socket.io@2.0.0@egg-socket.io/lib/io.js:55:5)
    at AppWorkerLoader.loadFile (/root/projects/agent-server/node_modules/_egg-core@3.7.0@egg-core/lib/loader/egg_loader.js:270:34)
    at getLoadUnits.forEach.unit (/root/projects/agent-server/node_modules/_egg-core@3.7.0@egg-core/lib/loader/mixin/custom.js:25:29)
    at Array.forEach (native)
    at AppWorkerLoader.loadCustomApp (/root/projects/agent-server/node_modules/_egg-core@3.7.0@egg-core/lib/loader/mixin/custom.js:25:8)
    at AppWorkerLoader.load (/root/projects/agent-server/node_modules/_egg@1.3.0@egg/lib/loader/app_worker_loader.js:33:10)
    at new Application (/root/projects/agent-server/node_modules/_egg@1.3.0@egg/lib/application.js:34:19)
    at Object.<anonymous> (/root/projects/agent-server/node_modules/_egg-cluster@1.6.2@egg-cluster/lib/app_worker.js:12:13)

这是什么原因呢？ node版本是6.10.2 npm版本3.10.10 egg-socket.io是直接用npm安装的（也用cnpm安装过，一样的错误）