TypeError: Cannot read property 'replace' of undefined

Hi,

First of all thank you. This library seems like a great option and I enjoyed your tutorial.

Now: when I try to `createIsomorphicWebpack(config);`

Compilation fails and I get the error above.

**server.js**
```
import path from 'path';
import webpack from 'webpack';
import express from 'express';
import config from './webpack.config';
import {
createIsomorphicWebpack
} from 'isomorphic-webpack';

// import render from './src/index.server.jsx';

const app = express();
const compiler = webpack(config);
// createIsomorphicWebpack(config);

app.set('views', './src/views');
app.set('view engine', 'ejs');

app.use(require('webpack-dev-middleware')(compiler, {
publicPath: config.output.publicPath
}));

app.use(require('webpack-hot-middleware')(compiler));

app.get('*', function(req, res) {

res.render('index', { html: '' });
});

app.listen(3000, function(err) {
if (err) {
return console.error(err);
}

console.log('Listening at http://localhost:3000/');
});
```

**webpack.config.js**
```
var path = require('path');
var webpack = require('webpack');
var UglifyJSPlugin = require('uglifyjs-webpack-plugin');

module.exports = {
devtool: 'cheap-module-source-map',
entry: {
app : [
'webpack-hot-middleware/client',
'babel-polyfill',
'./src/index.client.jsx'
],
vendor : [
'react',
'react-dom',
'react-router-dom'
]
},
output: {
path: path.join(__dirname, 'dist'),
filename: '[name].js',
chunkFilename: '[name].bundle.js',
publicPath: '/'
},
plugins: [
new webpack.HotModuleReplacementPlugin(),
new webpack.optimize.DedupePlugin(),
new webpack.optimize.OccurenceOrderPlugin(),
new UglifyJSPlugin({
parallel: true,
sourceMap: false,
uglifyOptions: {
ie8: false,
compress: true
}
}),
new webpack.optimize.CommonsChunkPlugin({
name: 'common' // Specify the common bundle's name.
})
],
module: {
loaders: [{
test: /\.(js|jsx)$/,
loaders: ['react-hot', 'babel'],
include: path.join(__dirname, 'src')
}, {
test: /\.css$/,
loader: 'style-loader'
}, {
test: /\.css$/,
loader: 'css-loader',
query: {
modules: true,
localIdentName: '[name]__[local]___[hash:base64:5]'
}
}, {
test: /\.scss$/,
loader: 'style-loader'
}, {
test: /\.scss$/,
loader: 'css-loader',
query: {
modules: true,
localIdentName: '[name]__[local]___[hash:base64:5]'
}
}, {
test: /\.scss$/,
loader: 'sass-loader',
query: {
outputStyle: 'expanded'
}
}, {
test: /\.(jpg|png|svg)$/,
loader: 'url-loader',
options: {
limit: 25000,
}
}]
}
};

```