diff --git a/libs/audio/include/psemek/audio/effect/envelope.hpp b/libs/audio/include/psemek/audio/effect/envelope.hpp
index 4056f082..23bd3934 100644
--- a/libs/audio/include/psemek/audio/effect/envelope.hpp
+++ b/libs/audio/include/psemek/audio/effect/envelope.hpp
@@ -1,3 +1,5 @@
+#pragma once
+
 #include <psemek/audio/stream.hpp>
 #include <psemek/audio/duration.hpp>