diff --git a/test/ruby/test_pack.rb b/test/ruby/test_pack.rb
index 7848eb5e62..621a06beba 100644
--- a/test/ruby/test_pack.rb
+++ b/test/ruby/test_pack.rb
@@ -445,7 +445,7 @@ class TestPack < Test::Unit::TestCase
       %w(f d e E g G).each do |f|
         v = [x].pack(f).unpack(f)
         if x.nan?
-          assert(v.first.nan?)
+          assert_predicate(v.first, :nan?)
         else
           assert_equal([x], v)
         end