diff --git a/activesupport/lib/active_support/inflector.rb b/activesupport/lib/active_support/inflector.rb
index dbe098bbccd9..97eb969d9955 100644
--- a/activesupport/lib/active_support/inflector.rb
+++ b/activesupport/lib/active_support/inflector.rb
@@ -281,9 +281,7 @@ def constantize(camel_cased_word)
       names.shift if names.empty? || names.first.empty?
 
       constant = Object
-      names.each do |name|
-        constant = constant.const_defined?(name) ? constant.const_get(name) : constant.const_missing(name)
-      end
+      names.each { |name| constant = constant.const_get(name) }
       constant
     end
 
diff --git a/activesupport/test/inflector_test.rb b/activesupport/test/inflector_test.rb
index 4ce9cbb705af..c586b456de95 100644
--- a/activesupport/test/inflector_test.rb
+++ b/activesupport/test/inflector_test.rb
@@ -2,8 +2,21 @@
 require 'inflector_test_cases'
 
 module Ace
+  module Extension
+    def self.included(base)
+      base.extend(ClassMethods)
+    end
+
+    module ClassMethods
+      def mission_accomplished?
+        false
+      end
+    end
+  end
+
   module Base
     class Case
+      include Extension
     end
   end
 end
@@ -121,7 +134,9 @@ def test_constantize
   end
 
   def test_constantize_does_lexical_lookup
-    assert_raises(NameError) { ActiveSupport::Inflector.constantize("Ace::Base::InflectorTest") }
+    assert_equal InflectorTest, ActiveSupport::Inflector.constantize("Ace::Base::InflectorTest")
+    assert_nothing_raised { Ace::Base::Case::ClassMethods }
+    assert_nothing_raised { assert_equal Ace::Base::Case::ClassMethods, ActiveSupport::Inflector.constantize("Ace::Base::Case::ClassMethods") }
   end
 
   def test_ordinal