Uncaught TypeError: Cannot read property '_currentElement'

"react": "^0.14.8",
ES6

ReactCompositeComponent.js:559 Uncaught TypeError: Cannot read property '_currentElement' of null

render()の時にエラーになっているようだった。
constructorに
this.state = {error: false}
を足したらエラーが消えた。

import React, { Component, PropTypes } from 'react'
import RaisedButton from 'material-ui/lib/raised-button';
import TextField from 'material-ui/lib/text-field';

const style = {
  margin: 12,
};

export default class Login extends Component {

  constructor(props, context){
    super(props);
    context.router // will work
    this.state = {error: false}
  }

  getInitialState() {
    return {
      error: false
    }
  }

  handleSubmit(event) {
    event.preventDefault()

    const email = this.refs.email.value
    const pass = this.refs.pass.value

    auth.login(email, pass, (loggedIn) => {
      if (!loggedIn)
        return this.setState({ error: true })

      const { location } = this.props

      if (location.state && location.state.nextPathname) {
        this.context.router.replace(location.state.nextPathname)
      } else {
        this.context.router.replace('/')
      }
    })
  }

  render() {
    return (
      <form onSubmit={this.handleSubmit}>
        <label><input ref="email" placeholder="email" defaultValue="joe@example.com" /></label>
        <label><input ref="pass" placeholder="password" /></label> (hint: password1)<br />
        <button type="submit">login</button>
        {this.state.error && (
          <p>Bad login information</p>
        )}
      </form>
    )
  }
}