diff --git a/actionview/lib/action_view/lookup_context.rb b/actionview/lib/action_view/lookup_context.rb
index 2eaf188ecf293c905da63b848d423c14ad070193..fd3d025cbf760f31c423de337c27a5c110cb4e55 100644
--- a/actionview/lib/action_view/lookup_context.rb
+++ b/actionview/lib/action_view/lookup_context.rb
@@ -280,7 +280,15 @@ def initialize_details(target, details)
     # add :html as fallback to :js.
     def formats=(values)
       if values
+        values = values.dup
         values.concat(default_formats) if values.delete "*/*"
+        values.uniq!
+
+        invalid_values = (values - Template::Types.symbols)
+        unless invalid_values.empty?
+          raise ArgumentError, "Invalid formats: #{invalid_values.map(&:inspect).join(", ")}"
+        end
+
         if values == [:js]
           values << :html
           @html_fallback_for_js = true
diff --git a/actionview/lib/action_view/rendering.rb b/actionview/lib/action_view/rendering.rb
index ac861c44d4ed99eae413fa268ddaa424de242e51..5a06bd9da6c16f1bd2fbc1f86c856bede23c5cfd 100644
--- a/actionview/lib/action_view/rendering.rb
+++ b/actionview/lib/action_view/rendering.rb
@@ -127,7 +127,7 @@ def _render_template(options)
       # Assign the rendered format to look up context.
       def _process_format(format)
         super
-        lookup_context.formats = [format.to_sym]
+        lookup_context.formats = [format.to_sym] if format.to_sym
       end
 
       # Normalize args by converting render "foo" to render :action => "foo" and
diff --git a/actionview/test/template/lookup_context_test.rb b/actionview/test/template/lookup_context_test.rb
index 537f8ee163b5bedeb72dde3ba4fea86100105fc4..3e357fe1a755ee160e5155dbaf5b559c3a130f34 100644
--- a/actionview/test/template/lookup_context_test.rb
+++ b/actionview/test/template/lookup_context_test.rb
@@ -67,7 +67,7 @@ def teardown
 
   test "handles explicitly defined */* formats fallback to :js" do
     @lookup_context.formats = [:js, Mime::ALL]
-    assert_equal [:js, *Mime::SET.symbols], @lookup_context.formats
+    assert_equal [:js, *Mime::SET.symbols].uniq, @lookup_context.formats
   end
 
   test "adds :html fallback to :js formats" do
@@ -75,6 +75,14 @@ def teardown
     assert_equal [:js, :html], @lookup_context.formats
   end
 
+  test "raises on invalid format assignment" do
+    ex = assert_raises ArgumentError do
+      @lookup_context.formats = [:html, :invalid, "also bad"]
+    end
+
+    assert_equal 'Invalid formats: :invalid, "also bad"', ex.message
+  end
+
   test "provides getters and setters for locale" do
     @lookup_context.locale = :pt
     assert_equal :pt, @lookup_context.locale
diff --git a/railties/lib/rails/mailers_controller.rb b/railties/lib/rails/mailers_controller.rb
index 95dae3ec2de7ea7381e06d61757c996fe4691b4b..5cffa52860c8ae8137b1bc73f9353204f21c30c6 100644
--- a/railties/lib/rails/mailers_controller.rb
+++ b/railties/lib/rails/mailers_controller.rb
@@ -38,7 +38,7 @@ def preview
           end
         else
           @part = find_preferred_part(request.format, Mime[:html], Mime[:text])
-          render action: "email", layout: false, formats: %w[html]
+          render action: "email", layout: false, formats: [:html]
         end
       else
         raise AbstractController::ActionNotFound, "Email '#{@email_action}' not found in #{@preview.name}"