diff --git a/libs/ardour/ardour/triggerbox.h b/libs/ardour/ardour/triggerbox.h
index cf1d67ad08..c3d9b2fc5a 100644
--- a/libs/ardour/ardour/triggerbox.h
+++ b/libs/ardour/ardour/triggerbox.h
@@ -72,7 +72,7 @@ class Session;
 class AudioRegion;
 class MidiRegion;
 class TriggerBox;
-class SlotArmInfo;
+struct SlotArmInfo;
 class SideChain;
 class MidiPort;