diff --git a/ChangeLog b/ChangeLog
index ce37c52dd2..6cbbe5348b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sun Jan 30 09:37:25 2011  Yutaka Kanemoto  <kanemoto@ruby-lang.org>
+
+	* io.c (struct argf): char behaves like an unsigned char
+	  by default on AIX.
+
 Sun Jan 30 08:02:55 2011  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* configure.in: Mac OS X wrongly reports it has fdatasync(3).
diff --git a/io.c b/io.c
index 17784588bb..0b0a278697 100644
--- a/io.c
+++ b/io.c
@@ -150,7 +150,7 @@ struct argf {
     VALUE argv;
     char *inplace;
     struct rb_io_enc_t encs;
-    char init_p, next_p, binmode;
+    int8_t init_p, next_p, binmode;
 };
 
 static int max_file_descriptor = NOFILE;