DictationRecognizer always fails with UnknownError

I am using Windows 10 and trying to use the DictationRecognizer.

It always results in an UnknownError. I'm at a loss for what to try to fix it.

Cortana works on my machine and understands dictation. I tried building with a Windows Store build but that didn't affect the result. The KeywordRecognizer works.

 Dictation completed unsuccessfully: UnknownError.
 UnityEngine.Debug:LogErrorFormat(String, Object[])
 DictationTest:<Start>m__5(DictationCompletionCause) (at Assets/DictationTest.cs:35)

Here's the code I'm using. It's adapted from these docs.

 using UnityEngine;
 using System.Collections;
 using UnityEngine.Windows.Speech;
 
 public class DictationTest : MonoBehaviour
 {
 
     private DictationRecognizer m_DictationRecognizer;
 
     void Start()
     {
 
         Debug.Log ("phrase rec sup" + PhraseRecognitionSystem.isSupported);
 
         m_DictationRecognizer = new DictationRecognizer(DictationTopicConstraint.Form);
 
         m_DictationRecognizer.DictationResult += (text, confidence) =>
         {
             Debug.LogFormat("Dictation result: {0}", text);
         };
 
         m_DictationRecognizer.DictationHypothesis += (text) =>
         {
             Debug.LogFormat("Dictation hypothesis: {0}", text);
         };
 
         m_DictationRecognizer.DictationComplete += (completionCause) =>
         {
             if (completionCause != DictationCompletionCause.Complete)
                 Debug.LogErrorFormat("Dictation completed unsuccessfully: {0}.", completionCause);
         };
 
         m_DictationRecognizer.DictationError += (error, hresult) =>
         {
             Debug.LogErrorFormat("Dictation error: {0}; HResult = {1}.", error, hresult);
         };
 
         m_DictationRecognizer.Start();
     }
 }