diff --git a/ChangeLog b/ChangeLog
index 5ad1105e52..a39f2dc505 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,14 @@
+Mon Sep 26 15:43:34 2016  Kazuki Yamaguchi  <k@rhe.jp>
+
+	* eval_intern.h (TH_PUSH_TAG): Initialize struct rb_vm_tag::tag with
+	  Qundef rather than 0 which is equal to Qfalse. Since Kernel#throw(obj)
+	  searches a tag with rb_vm_tag::tag == obj, throw(false) can
+	  accidentally find an unrelated tag which is not created by
+	  Kernel#catch.  [ruby-core:77229] [Bug #12743]
+
+	* test/ruby/test_exception.rb (test_throw_false): Add a test case for
+	  this.
+
 Mon Sep 26 14:36:12 2016  Naotoshi Seo  <sonots@gmail.com>
 
 	* lib/tempfile.rb: provide default basename parameter for
diff --git a/eval_intern.h b/eval_intern.h
index 9db0fd1bdb..c5210b14de 100644
--- a/eval_intern.h
+++ b/eval_intern.h
@@ -131,7 +131,7 @@ LONG WINAPI rb_w32_stack_overflow_handler(struct _EXCEPTION_POINTERS *);
 #define TH_PUSH_TAG(th) do { \
   rb_thread_t * const _th = (th); \
   struct rb_vm_tag _tag; \
-  _tag.tag = 0; \
+  _tag.tag = Qundef; \
   _tag.prev = _th->tag;
 
 #define TH_POP_TAG() \
diff --git a/test/ruby/test_exception.rb b/test/ruby/test_exception.rb
index 10489dd5ba..4da3de165b 100644
--- a/test/ruby/test_exception.rb
+++ b/test/ruby/test_exception.rb
@@ -181,6 +181,15 @@ class TestException < Test::Unit::TestCase
     }
   end
 
+  def test_throw_false
+    bug12743 = '[ruby-core:77229] [Bug #12743]'
+    assert_raise_with_message(UncaughtThrowError, /false/, bug12743) {
+      Thread.start {
+        throw false
+      }.join
+    }
+  end
+
   def test_else_no_exception
     begin
       assert(true)