InvalidOperationException in System.speech (Simple speech dictation program)

The first time I start my program, it worked fine for the first two words I spoke before it stopped inputting text. The Exception error is shown on:

recognitionEngine.RecognizeAsync(RecognizeMode.Multiple)

" Cannot perform this operation while the recognizer is doing recognition."

Now, anytime I start my program it does not type anything I say, and I've no idea where to find the problem. the following is the rest of the code in the program, in case I've overlooked an Error.

public partial class Form1 : Form
{
    private SpeechRecognitionEngine recognitionEngine;
    public Form1()
    {
        InitializeComponent();

        recognitionEngine = new SpeechRecognitionEngine();
        recognitionEngine.SetInputToDefaultAudioDevice();
        recognitionEngine.SpeechRecognized += (s, args) =>
            {
                foreach (RecognizedWordUnit word in args.Result.Words)
                {
                    if (word.Confidence > 0.8f)
                        richTextBox1.Text += word.Text + " ";
                }
                richTextBox1.Text += Environment.NewLine;
            };
        recognitionEngine.LoadGrammar(new DictationGrammar());
    }

    private void button1_Click(object sender, EventArgs e)
    {
        recognitionEngine.RecognizeAsync(RecognizeMode.Multiple);
    }

    private void button2_Click(object sender, EventArgs e)
    {
        recognitionEngine.RecognizeAsyncStop();
    }
}