diff --git a/include/mrbconf.h b/include/mrbconf.h
index 0caa3f90e..988bdf0ad 100644
--- a/include/mrbconf.h
+++ b/include/mrbconf.h
@@ -8,6 +8,7 @@
 #define MRUBYCONF_H
 
 #include <stdint.h>
+#include <stddef.h>
 
 /* configuration options: */
 /* add -DMRB_USE_FLOAT to use float instead of double for floating point numbers */