diff --git a/libs/ardour/ardour/audio_buffer.h b/libs/ardour/ardour/audio_buffer.h
index faa3de7685..6b771814db 100644
--- a/libs/ardour/ardour/audio_buffer.h
+++ b/libs/ardour/ardour/audio_buffer.h
@@ -173,6 +173,7 @@ public:
 
 	void prepare () { _written = false; _silent = false; }
 	bool written() const { return _written; }
+	void set_written(bool w) { _written = w; }
 
   private:
 	bool    _owns_data;
diff --git a/libs/ardour/port_insert.cc b/libs/ardour/port_insert.cc
index 05d0aa3c6f..aa2d1392f5 100644
--- a/libs/ardour/port_insert.cc
+++ b/libs/ardour/port_insert.cc
@@ -117,6 +117,7 @@ PortInsert::run (BufferSet& bufs, framepos_t start_frame, framepos_t end_frame,
                         _mtdm->process (nframes, in, out);
 			
                         outbuf.set_is_silent (false);
+                        outbuf.set_written (true);
                 }
 
                 return;