diff --git a/ChangeLog b/ChangeLog
index 34ce1956d3..543c13db83 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Sun May 15 21:22:35 2011  CHIKANAGA Tomoyuki  <nagachika00@gmail.com>
+
+	* cont.c (cont_init): clear macihne_stack_start/end of saved thread to
+	  prevent mark machine stack of GC'ed Thread. root Fiber is not initialized by
+	  fiber_init(). based on a patch by Serge Balyuk [ruby-core:35891] fixes #4612
+	* test/ruby/test_fiber.rb (test_gc_root_fiber): add test for it.
+
 Sun May 15 21:04:29 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* transcode.c (econv_init): revert r31353.  [ruby-dev:43512]
diff --git a/cont.c b/cont.c
index ad277bda1d..9a0593c21e 100644
--- a/cont.c
+++ b/cont.c
@@ -383,6 +383,7 @@ cont_init(rb_context_t *cont, rb_thread_t *th)
     /* save thread context */
     cont->saved_thread = *th;
     cont->saved_thread.local_storage = 0;
+    cont->saved_thread.machine_stack_start = cont->saved_thread.machine_stack_end = 0;
 }
 
 static rb_context_t *
diff --git a/test/ruby/test_fiber.rb b/test/ruby/test_fiber.rb
index ec64f98229..e3e40566f8 100644
--- a/test/ruby/test_fiber.rb
+++ b/test/ruby/test_fiber.rb
@@ -197,5 +197,16 @@ class TestFiber < Test::Unit::TestCase
       end.join
     end
   end
+
+  def test_gc_root_fiber
+    bug4612 = '[ruby-core:35891]'
+
+    assert_normal_exit %q{
+      require 'fiber'
+      GC.stress = true
+      Thread.start{ Fiber.current; nil }.join
+      GC.start
+    }, bug4612
+  end
 end