diff --git a/lib/haml/buffer.rb b/lib/haml/buffer.rb
index 8f066601..3906296c 100644
--- a/lib/haml/buffer.rb
+++ b/lib/haml/buffer.rb
@@ -123,11 +123,15 @@ module Haml
     def self.merge_attrs(to, from)
       if to['id'] && from['id']
         to['id'] << '_' << from.delete('id')
+      elsif to['id'] || from['id']
+        from['id'] ||= to['id']
       end
 
       if to['class'] && from['class']
         # Make sure we don't duplicate class names
         from['class'] = (from['class'].split(' ') | to['class'].split(' ')).join(' ')
+      elsif to['class'] || from['class']
+        from['class'] ||= to['class']
       end
 
       to.merge!(from)
diff --git a/test/haml/engine_test.rb b/test/haml/engine_test.rb
index de99fb8d..b1850017 100644
--- a/test/haml/engine_test.rb
+++ b/test/haml/engine_test.rb
@@ -139,6 +139,19 @@ class EngineTest < Test::Unit::TestCase
     assert_equal("<p>nil</p>\n", render("%p{ :attr => x } nil", :locals => {:x => nil}))
   end
 
+  def test_nil_id_with_syntactic_id
+    assert_equal("<p id='foo'>nil</p>\n", render("%p#foo{:id => nil} nil"))
+    assert_equal("<p id='foo_bar'>nil</p>\n", render("%p#foo{{:id => 'bar'}, :id => nil} nil"))
+    assert_equal("<p id='foo_bar'>nil</p>\n", render("%p#foo{{:id => nil}, :id => 'bar'} nil"))
+  end
+
+  def test_nil_class_with_syntactic_class
+    assert_equal("<p class='foo'>nil</p>\n", render("%p.foo{:class => nil} nil"))
+    assert_equal("<p class='bar foo'>nil</p>\n", render("%p.bar.foo{:class => nil} nil"))
+    assert_equal("<p class='bar foo'>nil</p>\n", render("%p.foo{{:class => 'bar'}, :class => nil} nil"))
+    assert_equal("<p class='bar foo'>nil</p>\n", render("%p.foo{{:class => nil}, :class => 'bar'} nil"))
+  end
+
   def test_locals
     assert_equal("<p>Paragraph!</p>\n", render("%p= text", :locals => { :text => "Paragraph!" }))
   end