Unity中的SpeechSynthesizer出现空引用异常

我在Unity项目中使用C语言合成器,如以下示例所示:

https://docs.microsoft.com/en-us/dotnet/api/system.speech.synthesis.speechsynthesizer?view=netframework-4.8

我得到这个错误:

NullReferenceException: Object reference not set to an instance of an object
System.Speech.Internal.ObjectTokens.RegistryDataKey.HKEYfromRegKey (Microsoft.Win32.RegistryKey regKey) (at [...])
System.Speech.Internal.ObjectTokens.RegistryDataKey.RootHKEYFromRegPath (System.String rootPath) (at [...])
System.Speech.Internal.ObjectTokens.RegistryDataKey.Open (System.String registryPath, System.Boolean fCreateIfNotExist) (at [...])
System.Speech.Internal.ObjectTokens.ObjectTokenCategory.Create (System.String sCategoryId) (at [...])
System.Speech.Internal.ObjectTokens.SAPICategories.DefaultDeviceOut () (at [...])
System.Speech.Internal.Synthesis.VoiceSynthesis..ctor (System.WeakReference speechSynthesizer) (at [...])
System.Speech.Synthesis.SpeechSynthesizer.get_VoiceSynthesizer () (at [...])
System.Speech.Synthesis.SpeechSynthesizer.SelectVoiceByHints (System.Speech.Synthesis.VoiceGender gender, System.Speech.Synthesis.VoiceAge age, System.Int32 voiceAlternate, System.Globalization.CultureInfo culture) (at [...])
System.Speech.Synthesis.SpeechSynthesizer.SelectVoiceByHints (System.Speech.Synthesis.VoiceGender gender) (at [...])

这是我的课:

using System;
using System.Speech.Synthesis;
using UnityEngine;

    public class Speaker : MonoBehaviour
    {
        private SpeechSynthesizer synth = new SpeechSynthesizer();

        void Start() {        
            synth.SetOutputToDefaultAudioDevice();  
            synth.SelectVoiceByHints(VoiceGender.Female);
        }

      }

欢迎提出任何意见。