1 files changed, 5 insertions, 3 deletions
diff --git a/activesupport/lib/active_support/core_ext/hash/keys.rb b/activesupport/lib/active_support/core_ext/hash/keys.rb
index f35c2be4c2..93e716585b 100644
--- a/activesupport/lib/active_support/core_ext/hash/keys.rb
+++ b/activesupport/lib/active_support/core_ext/hash/keys.rb
@@ -61,13 +61,15 @@ class Hash
   # on a mismatch. Note that keys are NOT treated indifferently, meaning if you
   # use strings for keys but assert symbols as keys, this will fail.
   #
-  #   { 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', years: '28' }.assert_valid_keys(:name, :age) # => raises "ArgumentError: Unknown key: :years. Valid keys are: :name, :age"
+  #   { name: 'Rob', age: '28' }.assert_valid_keys('name', 'age') # => raises "ArgumentError: Unknown key: :name. Valid keys are: 'name', 'age'"
   #   { name: 'Rob', age: '28' }.assert_valid_keys(:name, :age)   # => passes, raises nothing
   def assert_valid_keys(*valid_keys)
     valid_keys.flatten!
     each_key do |k|
-      raise ArgumentError.new("Unknown key: #{k}") unless valid_keys.include?(k)
+      unless valid_keys.include?(k)
+        raise ArgumentError.new("Unknown key: #{k.inspect}. Valid keys are: #{valid_keys.map(&:inspect).join(', ')}")
+      end
     end
   end