diff --git a/src/basic_serializer_map.cpp b/src/basic_serializer_map.cpp
index 5506aa79..346c4159 100644
--- a/src/basic_serializer_map.cpp
+++ b/src/basic_serializer_map.cpp
@@ -12,6 +12,8 @@
 #  pragma warning (disable : 4786) // too long name, harmless warning
 #endif
 
+#include <stdlib.h> // for NULL
+
 #include <set>
 
 #define BOOST_ARCHIVE_SOURCE