diff --git a/src/lib/filters/threaded_fork.cpp b/src/lib/filters/threaded_fork.cpp
index 170264353e..d379eacda2 100644
--- a/src/lib/filters/threaded_fork.cpp
+++ b/src/lib/filters/threaded_fork.cpp
@@ -12,6 +12,7 @@
 
 #include <botan/internal/semaphore.h>
 #include <botan/internal/barrier.h>
+#include <functional>
 
 namespace Botan {