diff --git a/include/toml11/spec.hpp b/include/toml11/spec.hpp
index 4878ad3..df9f3d7 100644
--- a/include/toml11/spec.hpp
+++ b/include/toml11/spec.hpp
@@ -2,6 +2,7 @@
 #define TOML11_SPEC_HPP
 
 #include <array>
+#include <functional>
 #include <ostream>
 #include <sstream>
 #include <utility>