diff --git a/libs/ardour/ardour/polarity_processor.h b/libs/ardour/ardour/polarity_processor.h
index 0f0e8c27a0..136a726033 100644
--- a/libs/ardour/ardour/polarity_processor.h
+++ b/libs/ardour/ardour/polarity_processor.h
@@ -24,6 +24,8 @@
 
 namespace ARDOUR {
 
+class PhaseControl;
+
 class LIBARDOUR_API PolarityProcessor : public Processor
 {
 public: