2 files changed, 7 insertions, 5 deletions
diff --git a/activesupport/lib/active_support/core_ext/hash/keys.rb b/activesupport/lib/active_support/core_ext/hash/keys.rb
index 2cb2c89d30..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
diff --git a/activesupport/test/core_ext/hash_ext_test.rb b/activesupport/test/core_ext/hash_ext_test.rb
index fc8d8170a1..e5438745e0 100644
--- a/activesupport/test/core_ext/hash_ext_test.rb
+++ b/activesupport/test/core_ext/hash_ext_test.rb
@@ -282,7 +282,7 @@ class HashExtTest < Test::Unit::TestCase
       { :failure => "stuff", :funny => "business" }.assert_valid_keys(:failure, :funny)
     end
 
-    assert_raise(ArgumentError, "Unknown key(s): failore") do
+    assert_raise(ArgumentError, "Unknown key: failore") do
       { :failore => "stuff", :funny => "business" }.assert_valid_keys([ :failure, :funny ])
       { :failore => "stuff", :funny => "business" }.assert_valid_keys(:failure, :funny)
     end