diff --git a/ChangeLog b/ChangeLog
index ec4b409322..c9e0005fbd 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu Feb 12 17:17:51 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* ext/stringio/stringio.c (strio_ungetc): calculates new position
+	  before reallocation.  [Bug#1099]
+
 Thu Feb 12 16:50:27 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* configure.in: a patch to build on GNU/kOpenSolaris from Rober
diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 4ff0e36c69..ce1d55fd46 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -728,8 +728,9 @@ strio_ungetc(VALUE self, VALUE c)
 	p += clen;
 	lpos++;
     }
+    clen = p - RSTRING_PTR(ptr->string);
     rb_str_update(ptr->string, lpos, ptr->pos ? 1 : 0, c);
-    ptr->pos = p - RSTRING_PTR(ptr->string);
+    ptr->pos = clen;
 
     return Qnil;
 }