diff --git a/src/effects/Effect.cpp b/src/effects/Effect.cpp
index 4748887e3..f0be05ed6 100644
--- a/src/effects/Effect.cpp
+++ b/src/effects/Effect.cpp
@@ -1282,6 +1282,11 @@ bool Effect::Process()
    CopyInputTracks(Track::All);
    bool bGoodResult = true;
 
+   // It's possible that the number of channels the effect expects changed based on
+   // the parameters (the Audacity Reverb effect does when the stereo width is 0).
+   mNumAudioIn = GetAudioInCount();
+   mNumAudioOut = GetAudioOutCount();
+
    mPass = 1;
    if (InitPass1())
    {
diff --git a/src/effects/Reverb.cpp b/src/effects/Reverb.cpp
index ece77c13b..e026aeca4 100644
--- a/src/effects/Reverb.cpp
+++ b/src/effects/Reverb.cpp
@@ -151,12 +151,12 @@ EffectType EffectReverb::GetType()
 
 int EffectReverb::GetAudioInCount()
 {
-   return 2;
+   return mParams.mStereoWidth ? 2 : 1;
 }
 
 int EffectReverb::GetAudioOutCount()
 {
-   return 2;
+   return mParams.mStereoWidth ? 2 : 1;
 }
 
 #define BLOCK 16384
@@ -230,7 +230,7 @@ sampleCount EffectReverb::ProcessBlock(float **inBlock, float **outBlock, sample
          reverb_process(&mP[c].reverb, len);
       }
 
-      if (mNumChans == 2 && mParams.mStereoWidth)
+      if (mNumChans == 2)
       {
          for (sampleCount i = 0; i < len; i++)
          {