Module build failed: TypeError: Cannot read property 'parse' of undefined

Hey, I'm trying to set up this plugin in my project, but Webpack compilation fails with the following error:

```
ERROR in ./app/client/silvertip/elements/progress_bar.js
Module build failed: TypeError: /Users/tim/Development/Rails/go/app/client/silvertip/elements/progress_bar.js: Cannot read property 'parse' of undefined
at new LazyResult (/Users/tim/Development/Rails/go/node_modules/postcss/lib/lazy-result.js:63:23)
at Processor.process (/Users/tim/Development/Rails/go/node_modules/postcss/lib/processor.js:117:12)
at getTokens (/Users/tim/Development/Rails/go/node_modules/babel-plugin-react-css-modules/dist/requireCssModule.js:82:29)
at exports.default (/Users/tim/Development/Rails/go/node_modules/babel-plugin-react-css-modules/dist/requireCssModule.js:125:10)
at PluginPass.ImportDeclaration (/Users/tim/Development/Rails/go/node_modules/babel-plugin-react-css-modules/dist/index.js:156:102)
at newFn (/Users/tim/Development/Rails/go/node_modules/babel-traverse/lib/visitors.js:276:21)
at NodePath._call (/Users/tim/Development/Rails/go/node_modules/babel-traverse/lib/path/context.js:76:18)
at NodePath.call (/Users/tim/Development/Rails/go/node_modules/babel-traverse/lib/path/context.js:48:17)
at NodePath.visit (/Users/tim/Development/Rails/go/node_modules/babel-traverse/lib/path/context.js:105:12)
at TraversalContext.visitQueue (/Users/tim/Development/Rails/go/node_modules/babel-traverse/lib/context.js:150:16)
```

package.json:

``` json
{
"engines": {
"node": "6.x",
"yarn": "1.2.1"
},
"dependencies": {
"@rails/webpacker": "^3.0.2",
"babel-plugin-react-css-modules": "^3.3.2",
"babel-preset-react": "^6.24.1",
"react": "^15.6.1",
"react-dom": "^15.6.1"
},
"devDependencies": {
"babel-jest": "20.0.1",
"enzyme": "^3.1.0",
"enzyme-adapter-react-15": "^1.0.3",
"eslint": "4.9.0",
"eslint-config-airbnb": "^16.1.0",
"eslint-loader": "^1.9.0",
"eslint-plugin-import": "^2.8.0",
"eslint-plugin-jsx-a11y": "^6.0.2",
"eslint-plugin-react": "^7.4.0",
"identity-obj-proxy": "^3.0.0",
"jest": "20.0.1",
"postcss-cssnext": "^3.0.2",
"postcss-sass": "^0.2.0",
"postcss-scss": "^1.0.2",
"precss": "^2.0.0",
"react-test-renderer": "^15.6.1",
"webpack-dev-server": "^2.9.4",
"webpack-merge": "^4.1.1"
}
}
```

.babelrc:

``` json
{
"presets": [
"env",
"react"
],
"plugins": [
"syntax-dynamic-import",
"transform-object-rest-spread",
[
"transform-class-properties",
{
"spec": true
}
],
[
"react-css-modules",
{
"exclude": "node_modules",
"webpackHotModuleReloading": true,
"generateScopedName": "[path]___[name]__[local]___[hash:base64:5]",
"filetypes": {
".sass": {
"syntax": "postcss-sass",
"plugins": ["postcss-nested"]
},
".scss": {
"syntax": "postcss-scss",
"plugins": ["postcss-nested"]
}
}
}
]
]
}
```

Abbreviated JS source file:

``` javascript
import React from 'react'
import './progress_bar.sass'

class ProgressBar extends React.Component {
render() {
return
}
}
```

The Webpack configuration comes from the `@rails/webpacker` node module. Any help or a direction I should investigate is appreciated. Thanks!

Beginne mit app/client/silvertip/elements/progress_bar.js und reproduziere den Webpack-Fehler mithilfe der gezeigten package.json- und .babelrc-Konfiguration. Verfolge die Verarbeitung von progress_bar.sass durch react-css-modules und überprüfe, dass der Build ohne den gemeldeten TypeError abgeschlossen wird.

Vom Indexierungsmodell aus dem Issue-Text verfasst.