diff --git a/actionpack/lib/action_view/helpers/asset_tag_helpers/javascript_tag_helpers.rb b/actionpack/lib/action_view/helpers/asset_tag_helpers/javascript_tag_helpers.rb
index b9126af9440de647b7bd58dd1f07edb114556250..82bbfcc7d22f3bd53134ff6d933f8f088abf4280 100644
--- a/actionpack/lib/action_view/helpers/asset_tag_helpers/javascript_tag_helpers.rb
+++ b/actionpack/lib/action_view/helpers/asset_tag_helpers/javascript_tag_helpers.rb
@@ -69,7 +69,7 @@ module ClassMethods
           def register_javascript_expansion(expansions)
             js_expansions = JavascriptIncludeTag.expansions
             expansions.each do |key, values|
-              js_expansions[key] = (js_expansions[key] || []) | Array(values) if values
+              js_expansions[key] = (js_expansions[key] || []) | Array(values)
             end
           end
         end
diff --git a/actionpack/lib/action_view/helpers/asset_tag_helpers/stylesheet_tag_helpers.rb b/actionpack/lib/action_view/helpers/asset_tag_helpers/stylesheet_tag_helpers.rb
index f3e041de95bd08758901567d3d156942f42b4166..a48c87b49ab52dd1ca0ab790dfe5109e69bef021 100644
--- a/actionpack/lib/action_view/helpers/asset_tag_helpers/stylesheet_tag_helpers.rb
+++ b/actionpack/lib/action_view/helpers/asset_tag_helpers/stylesheet_tag_helpers.rb
@@ -46,7 +46,7 @@ module ClassMethods
           def register_stylesheet_expansion(expansions)
             style_expansions = StylesheetIncludeTag.expansions
             expansions.each do |key, values|
-              style_expansions[key] = (style_expansions[key] || []) | Array(values) if values
+              style_expansions[key] = (style_expansions[key] || []) | Array(values)
             end
           end
         end
diff --git a/actionpack/test/template/asset_tag_helper_test.rb b/actionpack/test/template/asset_tag_helper_test.rb
index f3f628f31bc395d1772f0440863407e688ae4c39..1bf748af14b8653c3fdf5617d0e132fab692e011 100644
--- a/actionpack/test/template/asset_tag_helper_test.rb
+++ b/actionpack/test/template/asset_tag_helper_test.rb
@@ -302,9 +302,13 @@ def test_registering_javascript_expansions_merges_with_existing_expansions
     assert_dom_equal  %(<script src="/javascripts/bank.js" type="text/javascript"></script>\n<script src="/javascripts/robber.js" type="text/javascript"></script>), javascript_include_tag(:can_merge)
   end
 
+  def test_custom_javascript_expansions_with_undefined_symbol
+    assert_raise(ArgumentError) { javascript_include_tag('first', :unknown, 'last') }
+  end
+
   def test_custom_javascript_expansions_with_nil_value
     ActionView::Helpers::AssetTagHelper::register_javascript_expansion :monkey => nil
-    assert_raise(ArgumentError) { javascript_include_tag('first', :monkey, 'last') }
+    assert_dom_equal  %(<script src="/javascripts/first.js" type="text/javascript"></script>\n<script src="/javascripts/last.js" type="text/javascript"></script>), javascript_include_tag('first', :monkey, 'last')
   end
 
   def test_custom_javascript_expansions_with_empty_array_value
@@ -384,9 +388,13 @@ def test_single_stylesheet_asset_keys_should_take_precedence_over_expansions
     assert_dom_equal  %(<link href="/stylesheets/london.css" media="screen" rel="stylesheet" type="text/css" />\n<link href="/stylesheets/wellington.css" media="screen" rel="stylesheet" type="text/css" />\n<link href="/stylesheets/amsterdam.css" media="screen" rel="stylesheet" type="text/css" />), stylesheet_link_tag('london', :cities)
   end
 
+  def test_custom_stylesheet_expansions_with_unknown_symbol
+    assert_raise(ArgumentError) { stylesheet_link_tag('first', :unknown, 'last') }
+  end
+
   def test_custom_stylesheet_expansions_with_nil_value
     ActionView::Helpers::AssetTagHelper::register_stylesheet_expansion :monkey => nil
-    assert_raise(ArgumentError) { stylesheet_link_tag('first', :monkey, 'last') }
+    assert_dom_equal  %(<link href="/stylesheets/first.css" rel="stylesheet" type="text/css" media="screen" />\n<link href="/stylesheets/last.css" rel="stylesheet" type="text/css" media="screen" />), stylesheet_link_tag('first', :monkey, 'last')
   end
 
   def test_custom_stylesheet_expansions_with_empty_array_value