diff --git a/activesupport/lib/active_support/inflector/methods.rb b/activesupport/lib/active_support/inflector/methods.rb
index 39648727fd..665545db3b 100644
--- a/activesupport/lib/active_support/inflector/methods.rb
+++ b/activesupport/lib/active_support/inflector/methods.rb
@@ -219,7 +219,12 @@ module ActiveSupport
     # unknown.
     def constantize(camel_cased_word)
       names = camel_cased_word.split('::')
-      names.shift if names.empty? || names.first.empty?
+
+      # Trigger a builtin NameError exception including the ill-formed constant in the message.
+      Object.const_get(camel_cased_word) if names.empty?
+
+      # Remove the first blank element in case of '::ClassName' notation.
+      names.shift if names.size > 1 && names.first.empty?
 
       names.inject(Object) do |constant, name|
         if constant == Object
diff --git a/activesupport/test/constantize_test_cases.rb b/activesupport/test/constantize_test_cases.rb
index 9b62295c96..bbeb710a0c 100644
--- a/activesupport/test/constantize_test_cases.rb
+++ b/activesupport/test/constantize_test_cases.rb
@@ -34,8 +34,6 @@ module ConstantizeTestCases
     assert_equal Case::Dice, yield("Object::Case::Dice")
     assert_equal ConstantizeTestCases, yield("ConstantizeTestCases")
     assert_equal ConstantizeTestCases, yield("::ConstantizeTestCases")
-    assert_equal Object, yield("")
-    assert_equal Object, yield("::")
     assert_raises(NameError) { yield("UnknownClass") }
     assert_raises(NameError) { yield("UnknownClass::Ace") }
     assert_raises(NameError) { yield("UnknownClass::Ace::Base") }
@@ -45,6 +43,8 @@ module ConstantizeTestCases
     assert_raises(NameError) { yield("Ace::Base::ConstantizeTestCases") }
     assert_raises(NameError) { yield("Ace::Gas::Base") }
     assert_raises(NameError) { yield("Ace::Gas::ConstantizeTestCases") }
+    assert_raises(NameError) { yield("") }
+    assert_raises(NameError) { yield("::") }
   end
 
   def run_safe_constantize_tests_on
@@ -58,8 +58,8 @@ module ConstantizeTestCases
     assert_equal Case::Dice, yield("Object::Case::Dice")
     assert_equal ConstantizeTestCases, yield("ConstantizeTestCases")
     assert_equal ConstantizeTestCases, yield("::ConstantizeTestCases")
-    assert_equal Object, yield("")
-    assert_equal Object, yield("::")
+    assert_nil yield("")
+    assert_nil yield("::")
     assert_nil yield("UnknownClass")
     assert_nil yield("UnknownClass::Ace")
     assert_nil yield("UnknownClass::Ace::Base")