diff --git a/lib/uri/common.rb b/lib/uri/common.rb
index 971a684..57708e2 100644
--- a/lib/uri/common.rb
+++ b/lib/uri/common.rb
@@ -575,6 +575,7 @@ def self.encode_www_form(enum, enc=nil)
             str << '='
             str << encode_www_form_component(w, enc)
           end
+          str
         end.join('&')
       else
         str = encode_www_form_component(k, enc)
diff --git a/test/uri/test_common.rb b/test/uri/test_common.rb
index 5692640..98098bb 100644
--- a/test/uri/test_common.rb
+++ b/test/uri/test_common.rb
@@ -17,6 +17,11 @@ class Foo
     end
   end
 
+
+  def test_encode_www_form_preserves_nil_array_values
+    assert_equal("foo&foo=a&foo", URI.encode_www_form(foo: [nil, "a", nil]))
+  end
+
   def test_fallback_constants
     EnvUtil.suppress_warning do
       assert_raise(NameError) { URI::FOO }