diff --git a/include/stdio.h b/include/stdio.h
index 98b6431296ad..8cb07e366289 100644
--- a/include/stdio.h
+++ b/include/stdio.h
@@ -240,7 +240,7 @@ int	 getc __P((FILE *));
 int	 getchar __P((void));
 char	*gets __P((char *));
 #if !defined(_ANSI_SOURCE) && !defined(_POSIX_SOURCE)
-extern int sys_nerr;			/* perror(3) external variables */
+extern __const int sys_nerr;		/* perror(3) external variables */
 extern __const char *__const sys_errlist[];
 #endif
 void	 perror __P((const char *));