diff --git a/libs/ardour/ardour/audio_unit.h b/libs/ardour/ardour/audio_unit.h
index 3a01349dd3..55faf4a08e 100644
--- a/libs/ardour/ardour/audio_unit.h
+++ b/libs/ardour/ardour/audio_unit.h
@@ -216,7 +216,7 @@ class LIBARDOUR_API AUPlugin : public ARDOUR::Plugin
 	samplecnt_t input_offset;
 	samplecnt_t *cb_offsets;
 	BufferSet* input_buffers;
-	ChanMapping * input_map;
+	ChanMapping const * input_map;
 	samplecnt_t samples_processed;
 	uint32_t   audio_input_cnt;