diff --git a/headers/vmath.hpp/vmath_fwd.hpp b/headers/vmath.hpp/vmath_fwd.hpp
index 31749b6..3c0bcf3 100644
--- a/headers/vmath.hpp/vmath_fwd.hpp
+++ b/headers/vmath.hpp/vmath_fwd.hpp
@@ -10,6 +10,7 @@
 #include <cstddef>
 #include <cstdlib>
 
+#include <functional>
 #include <initializer_list>
 #include <iterator>
 #include <limits>