diff --git a/include/boost/integer/common_factor_rt.hpp b/include/boost/integer/common_factor_rt.hpp
index d86155d..d27f094 100644
--- a/include/boost/integer/common_factor_rt.hpp
+++ b/include/boost/integer/common_factor_rt.hpp
@@ -20,6 +20,9 @@
 #ifndef BOOST_NO_CXX11_HDR_TYPE_TRAITS
 #include <type_traits>
 #endif
+#ifdef BOOST_NO_CXX11_HDR_FUNCTIONAL
+#include <functional>
+#endif
 
 #if ((defined(BOOST_MSVC) && (BOOST_MSVC >= 1600)) || (defined(__clang__) && defined(__c2__)) || (defined(BOOST_INTEL) && defined(_MSC_VER))) && (defined(_M_IX86) || defined(_M_X64))
 #include <intrin.h>