diff --git a/ChangeLog b/ChangeLog
index 2b029da144..65b59b0d94 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Mon May  7 10:49:36 2012  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* ext/bigdecimal/bigdecimal.c (Init_bigdecimal): define IDs before
+	  they are used. [ruby-core:44900] [Bug #6406]
+
 Mon May  7 10:27:56 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* ext/digest/rmd160/rmd160.c (RMD160_Update): fix for huge data.
diff --git a/ext/bigdecimal/bigdecimal.c b/ext/bigdecimal/bigdecimal.c
index 041d33fd7d..a2d705ce20 100644
--- a/ext/bigdecimal/bigdecimal.c
+++ b/ext/bigdecimal/bigdecimal.c
@@ -2858,6 +2858,10 @@ Init_bigdecimal(void)
 {
     VALUE arg;
 
+    id_BigDecimal_exception_mode = rb_intern_const("BigDecimal.exception_mode");
+    id_BigDecimal_rounding_mode = rb_intern_const("BigDecimal.rounding_mode");
+    id_BigDecimal_precision_limit = rb_intern_const("BigDecimal.precision_limit");
+
     /* Initialize VP routines */
     VpInit(0UL);
 
@@ -3053,10 +3057,6 @@ Init_bigdecimal(void)
     rb_define_singleton_method(rb_mBigMath, "exp", BigMath_s_exp, 2);
     rb_define_singleton_method(rb_mBigMath, "log", BigMath_s_log, 2);
 
-    id_BigDecimal_exception_mode = rb_intern_const("BigDecimal.exception_mode");
-    id_BigDecimal_rounding_mode = rb_intern_const("BigDecimal.rounding_mode");
-    id_BigDecimal_precision_limit = rb_intern_const("BigDecimal.precision_limit");
-
     id_up = rb_intern_const("up");
     id_down = rb_intern_const("down");
     id_truncate = rb_intern_const("truncate");
diff --git a/test/bigdecimal/test_bigdecimal.rb b/test/bigdecimal/test_bigdecimal.rb
index 975983c029..9d160dafbe 100644
--- a/test/bigdecimal/test_bigdecimal.rb
+++ b/test/bigdecimal/test_bigdecimal.rb
@@ -1311,4 +1311,10 @@ class TestBigDecimal < Test::Unit::TestCase
     code = "exit(BigDecimal.new('10.0') == 10.0.to_d)"
     assert_ruby_status(%w[-rbigdecimal -rbigdecimal/util -rmathn -], code, bug6093)
   end
+
+  def test_bug6406
+    assert_in_out_err(%w[-rbigdecimal --disable-gems], <<-EOS, [], [])
+    Thread.current.keys.to_s
+    EOS
+  end
 end