diff --git a/ChangeLog b/ChangeLog
index 3b3e0bf905..b8ada561c4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Nov 12 10:59:49 2011  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* io.c (setup_narg): fix off by one bug.
+
 Sat Nov 12 10:56:43 2011  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 
 	* io.c (+setup_narg): factor out length calculation logic.
diff --git a/io.c b/io.c
index bb62d3de54..45931adacf 100644
--- a/io.c
+++ b/io.c
@@ -7959,13 +7959,12 @@ setup_narg(int cmd, VALUE *argp, int io_p)
 		len = 256;
 	    rb_str_modify(arg);
 
-	    if (len <= RSTRING_LEN(arg)) {
-		len = RSTRING_LEN(arg);
-	    }
-	    if (RSTRING_LEN(arg) < len) {
+	    /* expand for data + sentinel. */
+	    if (RSTRING_LEN(arg) < len+1) {
 		rb_str_resize(arg, len+1);
 	    }
-	    RSTRING_PTR(arg)[len] = 17;	/* a little sanity check here */
+	    /* a little sanity check here */
+	    RSTRING_PTR(arg)[RSTRING_LEN(arg) - 1] = 17;
 	    narg = (long)(SIGNED_VALUE)RSTRING_PTR(arg);
 	}
     }