1 files changed, 6 insertions, 4 deletions
diff --git a/activesupport/lib/active_support/core_ext/hash/keys.rb b/activesupport/lib/active_support/core_ext/hash/keys.rb
index 045a6944fa..d8748b1138 100644
--- a/activesupport/lib/active_support/core_ext/hash/keys.rb
+++ b/activesupport/lib/active_support/core_ext/hash/keys.rb
@@ -35,11 +35,13 @@ class Hash
   # as keys, this will fail.
   #
   # ==== Examples
-  #   { :name => "Rob", :years => "28" }.assert_valid_keys(:name, :age) # => raises "ArgumentError: Unknown key(s): years"
-  #   { :name => "Rob", :age => "28" }.assert_valid_keys("name", "age") # => raises "ArgumentError: Unknown key(s): name, age"
+  #   { :name => "Rob", :years => "28" }.assert_valid_keys(:name, :age) # => raises "ArgumentError: Unknown key: years"
+  #   { :name => "Rob", :age => "28" }.assert_valid_keys("name", "age") # => raises "ArgumentError: Unknown key: name"
   #   { :name => "Rob", :age => "28" }.assert_valid_keys(:name, :age) # => passes, raises nothing
   def assert_valid_keys(*valid_keys)
-    unknown_keys = keys - [valid_keys].flatten
-    raise(ArgumentError, "Unknown key(s): #{unknown_keys.join(", ")}") unless unknown_keys.empty?
+    valid_keys.flatten!
+    each_key do |k|
+      raise(ArgumentError, "Unknown key: #{k}") unless valid_keys.include?(k)
+    end
   end
 end