diff --git a/test/socket/test_ancdata.rb b/test/socket/test_ancdata.rb
index 8ba7fad2f0..0c1029f1d6 100644
--- a/test/socket/test_ancdata.rb
+++ b/test/socket/test_ancdata.rb
@@ -55,4 +55,4 @@ class TestSocketAncData < Test::Unit::TestCase
       assert(!ancdata.cmsg_is?(:IP, :PKTINFO))
     end
   end
-end
+end if defined? Socket::AncillaryData