diff --git a/lib/smmorphsourcemodule.cc b/lib/smmorphsourcemodule.cc
index 220376e8..2108871e 100644
--- a/lib/smmorphsourcemodule.cc
+++ b/lib/smmorphsourcemodule.cc
@@ -79,6 +79,12 @@ SimpleWavSetSource::rt_audio_block (size_t index, RTAudioBlock& out_block)
     }
 }
 
+void
+SimpleWavSetSource::set_portamento_freq (float freq)
+{
+  // FIXME: support formant correction?
+}
+
 MorphSourceModule::MorphSourceModule (MorphPlanVoice *voice) :
   MorphOperatorModule (voice)
 {
diff --git a/lib/smmorphsourcemodule.hh b/lib/smmorphsourcemodule.hh
index b982008c..a61f6bc8 100644
--- a/lib/smmorphsourcemodule.hh
+++ b/lib/smmorphsourcemodule.hh
@@ -23,6 +23,7 @@ public:
   void        retrigger (int channel, float freq, int midi_velocity) override;
   Audio      *audio() override;
   bool        rt_audio_block (size_t index, RTAudioBlock& out_block) override;
+  void        set_portamento_freq (float freq) override;
 };
 
 class MorphSourceModule : public MorphOperatorModule