diff --git a/das-dn/inc/ryFileDef.h b/das-dn/inc/ryFileDef.h
index 43f413ee..7940948d 100644
--- a/das-dn/inc/ryFileDef.h
+++ b/das-dn/inc/ryFileDef.h
@@ -2,6 +2,7 @@
 #define _RYFILE_DEFINE_H_
 
 #include "zjdtypes.h"
+#include <cstddef>
 #include <map>
 #include <unordered_map>