Perf.start is not a function

I install react-addons-perf and expose-loader in my project

here is my webpack config 

```
     {
        test: require.resolve("react-addons-perf"),
        use: [
          {
            loader: 'expose-loader',
            options: 'Perf'
          }
        ]
      }
```

and i assign Perf to window 

but run Perf.start is not a function ,is  undefined 

Why?