ReactJS Uncaught TypeError: Cannot read property 'concat' of undefined

I want to add an element into state everytime a Submit button is pressed. The problem is, everytime I press the submit button, it return

Uncaught TypeError: Cannot read property 'concat' of undefined error and I could not figure out why it is undefined.

class ChatRoom extends React.Component{
  constructor(props){
    super(props);
    this.state = {
      rooms: ['room1', 'room2'],
      chatRoom: ''
    }
  }

  handleAddRoom = (e) => {
    console.log('handleAddRoom', e);
    this.setState({
      rooms: this.state.notes.concat(e)
    })
  }
  handleSubmit(){
    var newRoom = ReactDOM.findDOMNode(this.refs.room).value;
    console.log(newRoom);
    ReactDOM.findDOMNode(this.refs.room).value = '';
    this.handleAddRoom(newRoom);
  }
  render(){
    console.log("rooms: ", this.state.rooms)
    return(
      <div>
        Hello from chat room!! Please enter new room:

        <input type="text" className="form-control" placeholder="Add a new note" ref="room" />
        <span className="input-group-btn">
          <button className="btn btn-default" type="button" onClick={this.handleSubmit.bind(this)}>Submit</button>
        </span>
      </div>
    )
  }
}

This is the component that is giving me problem. I put console log on various locations and they are all returning the right values. Somehow when it is trying to concat JS says it is undefined value. How can I concat the element into rooms state?