diff --git a/ChangeLog b/ChangeLog
index 6b420094c9..6df7c12d3a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat May 28 03:04:27 2011  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* io.c (fill_cbuf): return MORE_CHAR_SUSPENDED when cbuf is not empty.
+
 Sat May 28 02:22:48 2011  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 
 	* string.c (rb_str_bytesize): rb_str_bytesize() should use LONG2NUM().
diff --git a/io.c b/io.c
index 20d526dfe0..4e1945c549 100644
--- a/io.c
+++ b/io.c
@@ -1720,6 +1720,9 @@ fill_cbuf(rb_io_t *fptr, int ec_flags)
             }
         }
     }
+    if (cbuf_len0 != fptr->cbuf.len)
+	return MORE_CHAR_SUSPENDED;
+
     return MORE_CHAR_FINISHED;
 }