diff --git a/test/ruby/test_array.rb b/test/ruby/test_array.rb
index 4a9a58b723..137c124f60 100644
--- a/test/ruby/test_array.rb
+++ b/test/ruby/test_array.rb
@@ -911,16 +911,19 @@ class TestArray < Test::Unit::TestCase
     $, = ""
     a = @cls[1, 2]
     assert_equal("12", a.join)
+    assert_equal("12", a.join(nil))
     assert_equal("1,2", a.join(','))
 
     $, = ""
     a = @cls[1, 2, 3]
     assert_equal("123", a.join)
+    assert_equal("123", a.join(nil))
     assert_equal("1,2,3", a.join(','))
 
     $, = ":"
     a = @cls[1, 2, 3]
     assert_equal("1:2:3", a.join)
+    assert_equal("1:2:3", a.join(nil))
     assert_equal("1,2,3", a.join(','))
 
     $, = ""