diff --git a/include/json/config.h b/include/json/config.h
index 7201ba7..1b0581f 100644
--- a/include/json/config.h
+++ b/include/json/config.h
@@ -5,6 +5,7 @@
 
 #ifndef JSON_CONFIG_H_INCLUDED
 #define JSON_CONFIG_H_INCLUDED
+#include <stddef.h>
 
 /// If defined, indicates that json library is embedded in CppTL library.
 //# define JSON_IN_CPPTL 1