diff --git a/libs/ardour/audio_unit.cc b/libs/ardour/audio_unit.cc
index 428138ab54..f30686379a 100644
--- a/libs/ardour/audio_unit.cc
+++ b/libs/ardour/audio_unit.cc
@@ -455,7 +455,7 @@ AUPlugin::AUPlugin (const AUPlugin& other)
 
 {
 	init ();
-	for (int i = 0; i < descriptors.size(); ++i) {
+	for (size_t i = 0; i < descriptors.size(); ++i) {
 		set_parameter (i, other.get_parameter (i));
 	}
 }