diff --git a/include/mp/util.h b/include/mp/util.h
index 6fc92778..66491477 100644
--- a/include/mp/util.h
+++ b/include/mp/util.h
@@ -7,6 +7,7 @@
 
 #include <capnp/schema.h>
 #include <cstddef>
+#include <functional>
 #include <future>
 #include <kj/common.h>
 #include <kj/exception.h>