diff --git a/ChangeLog b/ChangeLog
index b904d8c1b3..4d1b1d4c5b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Fri Aug 22 20:23:54 2014  Koichi Sasada  <ko1@atdot.net>
+
+	* string.c (rb_fstring): fix condition (easy to cause infinite loop!).
+
 Fri Aug 22 20:07:43 2014  Koichi Sasada  <ko1@atdot.net>
 
 	* string.c (rb_fstring, fstr_update_callback): simply delete garbage
@@ -17,7 +21,7 @@ Fri Aug 22 19:30:39 2014  SHIBATA Hiroshi  <shibata.hiroshi@gmail.com>
 
 Fri Aug 22 19:25:28 2014  SHIBATA Hiroshi  <shibata.hiroshi@gmail.com>
 
-	* test/ruby/test_complex.rb: fixed broken tests. Math is not
+	* test/ruby/test_complex.rb: fixed broken tests. Math is nota
 	  compatible CMath now.
 
 Fri Aug 22 15:36:09 2014  Nobuyoshi Nakada  <nobu@ruby-lang.org>
diff --git a/string.c b/string.c
index 1fcd3ccc85..b9863b8e25 100644
--- a/string.c
+++ b/string.c
@@ -232,7 +232,7 @@ rb_fstring(VALUE str)
     do {
 	ret = str;
 	st_update(frozen_strings, (st_data_t)str, fstr_update_callback, (st_data_t)&ret);
-    } while (ret != Qundef);
+    } while (ret == Qundef);
 
     return ret;
 }