diff --git a/libs/ardour/ardour/port_insert.h b/libs/ardour/ardour/port_insert.h
index f627dbf3e1..f69cf33162 100644
--- a/libs/ardour/ardour/port_insert.h
+++ b/libs/ardour/ardour/port_insert.h
@@ -58,9 +58,7 @@ public:
 
 	void run (BufferSet& bufs, samplepos_t start_sample, samplepos_t end_sample, double speed, pframes_t nframes, bool);
 
-	void flush_buffers (samplecnt_t nframes) {
-		_out->flush_buffers (nframes);
-	}
+	void flush_buffers (samplecnt_t nframes);
 
 	samplecnt_t signal_latency () const;
 
diff --git a/libs/ardour/port_insert.cc b/libs/ardour/port_insert.cc
index 37b0f7979b..7449c0d96e 100644
--- a/libs/ardour/port_insert.cc
+++ b/libs/ardour/port_insert.cc
@@ -204,6 +204,12 @@ PortInsert::run (BufferSet& bufs, samplepos_t start_sample, samplepos_t end_samp
 	}
 }
 
+void
+PortInsert::flush_buffers (samplecnt_t nframes)
+{
+	_out->flush_buffers (nframes);
+}
+
 XMLNode&
 PortInsert::state () const
 {