diff --git a/mrbgems/mruby-socket/src/socket.c b/mrbgems/mruby-socket/src/socket.c
index 7d9bfe99f..973425312 100644
--- a/mrbgems/mruby-socket/src/socket.c
+++ b/mrbgems/mruby-socket/src/socket.c
@@ -12,7 +12,9 @@
   #include <windows.h>
 
   #define SHUT_RDWR SD_BOTH
+  #ifndef _SSIZE_T_DEFINED
   typedef int ssize_t;
+  #endif
 #else
   #include <sys/types.h>
   #include <sys/socket.h>