diff --git a/synth/voice_allocator.h b/synth/voice_allocator.h
index 0009459..63d64ad 100644
--- a/synth/voice_allocator.h
+++ b/synth/voice_allocator.h
@@ -4,6 +4,7 @@
 #include <vector>
 #include <memory>
 #include <algorithm>
+#include <functional>
 
 namespace trnr {