diff --git a/src/id_generation.cpp b/src/id_generation.cpp
index 3b1f57d..6c3be07 100644
--- a/src/id_generation.cpp
+++ b/src/id_generation.cpp
@@ -6,6 +6,7 @@
     http://www.boost.org/LICENSE_1_0.txt)
 =============================================================================*/
 
+#include <cctype>
 #include "id_manager_impl.hpp"
 #include <boost/make_shared.hpp>
 #include <boost/unordered_map.hpp>