diff --git a/reactos/include/crt/sys/types.h b/reactos/include/crt/sys/types.h
index 026471fe1a3..cdd1d6e4d61 100644
--- a/reactos/include/crt/sys/types.h
+++ b/reactos/include/crt/sys/types.h
@@ -101,7 +101,10 @@ typedef _sigset_t	sigset_t;
 typedef long _ssize_t;
 
 #ifndef	_NO_OLDNAMES
+# ifndef _SSIZE_T_DEFINED
+#  define _SSIZE_T_DEFINED
 typedef _ssize_t ssize_t;
+# endif
 #endif
 #endif /* Not _SSIZE_T_ */