Index: lib/uri/common.rb
===================================================================
--- lib/uri/common.rb	(revision 39770)
+++ lib/uri/common.rb	(working copy)
@@ -971,12 +971,12 @@
   # See URI.decode_www_form_component, URI.encode_www_form
   def self.decode_www_form(str, enc=Encoding::UTF_8)
     return [] if str.empty?
-    unless /\A#{WFKV_}=#{WFKV_}(?:[;&]#{WFKV_}=#{WFKV_})*\z/o =~ str
+    unless /\A#{WFKV_}(=#{WFKV_})?(?:[;&]#{WFKV_}(=#{WFKV_})?)*\z/o =~ str
       raise ArgumentError, "invalid data of application/x-www-form-urlencoded (#{str})"
     end
     ary = []
-    $&.scan(/([^=;&]+)=([^;&]*)/) do
-      ary << [decode_www_form_component($1, enc), decode_www_form_component($2, enc)]
+    $&.scan(/([^=;&]+)(?:=([^;&]*))?/) do
+      ary << [decode_www_form_component($1, enc), (decode_www_form_component($2, enc) if $2)]
     end
     ary
   end
Index: test/uri/test_common.rb
===================================================================
--- test/uri/test_common.rb	(revision 39770)
+++ test/uri/test_common.rb	(working copy)
@@ -109,14 +109,14 @@
     assert_equal([%w[a 1], ["\u3042", "\u6F22"]],
                  URI.decode_www_form("a=1;%E3%81%82=%E6%BC%A2"))
     assert_equal([%w[?a 1], %w[a 2]], URI.decode_www_form("?a=1&a=2"))
+    assert_equal([['a', nil], ['b', nil]], URI.decode_www_form("a&b"))
     assert_equal([], URI.decode_www_form(""))
     assert_raise(ArgumentError){URI.decode_www_form("%=1")}
     assert_raise(ArgumentError){URI.decode_www_form("a=%")}
     assert_raise(ArgumentError){URI.decode_www_form("a=1&%=2")}
     assert_raise(ArgumentError){URI.decode_www_form("a=1&b=%")}
-    assert_raise(ArgumentError){URI.decode_www_form("a&b")}
-    bug4098 = '[ruby-core:33464]'
-    assert_raise(ArgumentError, bug4098){URI.decode_www_form("a=AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA&b")}
+    # ruby-core:33464 regression test
+    assert_equal([["a", "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"], ["b", nil]], URI.decode_www_form("a=AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA&b"))
   end
 end