diff --git a/src/PSOEncryptionSeedFinder.hh b/src/PSOEncryptionSeedFinder.hh
index 38713ac7..16c0d3c0 100644
--- a/src/PSOEncryptionSeedFinder.hh
+++ b/src/PSOEncryptionSeedFinder.hh
@@ -4,6 +4,7 @@
 #include <stddef.h>
 
 #include <atomic>
+#include <functional>
 #include <string>
 #include <thread>
 #include <utility>