diff --git a/mrbgems/mruby-io/src/io.c b/mrbgems/mruby-io/src/io.c
index 643671d1e..0abb9014a 100644
--- a/mrbgems/mruby-io/src/io.c
+++ b/mrbgems/mruby-io/src/io.c
@@ -25,6 +25,7 @@
   #include <io.h>
   #define open  _open
   #define close _close
+  #define dup _dup
   #define read  _read
   #define write _write
   #define lseek _lseek