diff --git a/activesupport/lib/active_support/core_ext/hash/keys.rb b/activesupport/lib/active_support/core_ext/hash/keys.rb
index 8e728691c6..e753e36124 100644
--- a/activesupport/lib/active_support/core_ext/hash/keys.rb
+++ b/activesupport/lib/active_support/core_ext/hash/keys.rb
@@ -7,7 +7,7 @@ class Hash
   #  # => { "NAME" => "Rob", "AGE" => "28" }
   def transform_keys
     result = {}
-    keys.each do |key|
+    each_key do |key|
       result[yield(key)] = self[key]
     end
     result