diff --git a/actionpack/lib/action_controller/metal/strong_parameters.rb b/actionpack/lib/action_controller/metal/strong_parameters.rb
index 99a4e312cf..386075bd30 100644
--- a/actionpack/lib/action_controller/metal/strong_parameters.rb
+++ b/actionpack/lib/action_controller/metal/strong_parameters.rb
@@ -124,10 +124,10 @@ module ActionController
     # <tt>ActionController::ParameterMissing</tt> error.
     #
     #   ActionController::Parameters.new(person: { name: 'Francesco' }).require(:person)
-    #   # => {"name"=>"Francesco"}
+    #   # => {"name"=>"Francesco"}
     #
     #   ActionController::Parameters.new(person: nil).require(:person)
-    #   # => ActionController::ParameterMissing: param not found: person
+    #   # => ActionController::ParameterMissing: param not found: person
     #
     #   ActionController::Parameters.new(person: {}).require(:person)
     #   # => ActionController::ParameterMissing: param not found: person
@@ -187,7 +187,7 @@ module ActionController
     #   # => {}
     #
     #   params.require(:person).permit(contact: :phone)
-    #   # => {"contact"=>{"phone"=>"555-1234"}}
+    #   # => {"contact"=>{"phone"=>"555-1234"}}
     #
     #   params.require(:person).permit(contact: [ :email, :phone ])
     #   # => {"contact"=>{"email"=>"none@test.com", "phone"=>"555-1234"}}
@@ -203,6 +203,8 @@ module ActionController
           end
           keys.grep(/\A#{Regexp.escape(filter)}\(\d+[if]?\)\z/) { |key| params[key] = self[key] }
         when Hash then
+          filter = filter.with_indifferent_access
+
           self.slice(*filter.keys).each do |key, values|
             return unless values
 
diff --git a/actionpack/test/controller/parameters/nested_parameters_test.rb b/actionpack/test/controller/parameters/nested_parameters_test.rb
index d287e79cba..6df849c4e2 100644
--- a/actionpack/test/controller/parameters/nested_parameters_test.rb
+++ b/actionpack/test/controller/parameters/nested_parameters_test.rb
@@ -36,6 +36,31 @@ class NestedParametersTest < ActiveSupport::TestCase
     assert_nil permitted[:magazine]
   end
 
+  test "permitted nested parameters with a string or a symbol as a key" do
+    params = ActionController::Parameters.new({
+      book: {
+        'authors' => [
+          { name: 'William Shakespeare', born: '1564-04-26' },
+          { name: 'Christopher Marlowe' }
+        ]
+      }
+    })
+
+    permitted = params.permit book: [ { 'authors' => [ :name ] } ]
+
+    assert_equal 'William Shakespeare', permitted[:book]['authors'][0][:name]
+    assert_equal 'William Shakespeare', permitted[:book][:authors][0][:name]
+    assert_equal 'Christopher Marlowe', permitted[:book]['authors'][1][:name]
+    assert_equal 'Christopher Marlowe', permitted[:book][:authors][1][:name]
+
+    permitted = params.permit book: [ { authors: [ :name ] } ]
+
+    assert_equal 'William Shakespeare', permitted[:book]['authors'][0][:name]
+    assert_equal 'William Shakespeare', permitted[:book][:authors][0][:name]
+    assert_equal 'Christopher Marlowe', permitted[:book]['authors'][1][:name]
+    assert_equal 'Christopher Marlowe', permitted[:book][:authors][1][:name]
+  end
+
   test "nested arrays with strings" do
     params = ActionController::Parameters.new({
       :book => {