diff --git a/ChangeLog b/ChangeLog
index affd0835ef..28b8bd7197 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Dec 20 17:49:03 2014  Tanaka Akira  <akr@fsij.org>
+
+	* lib/tmpdir.rb (Dir.mktmpdir): Accept nil again, as Ruby 2.1.
+	  [ruby-core:66943] [Bug #10616] Fixed by Alex Slynko.
+
 Sat Dec 20 11:22:58 2014  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* ext/fiddle/depend, ext/fiddle/extconf.rb: try to build bundled
diff --git a/lib/tmpdir.rb b/lib/tmpdir.rb
index f52d092de5..494725cb33 100644
--- a/lib/tmpdir.rb
+++ b/lib/tmpdir.rb
@@ -81,8 +81,8 @@ class Dir
   #    FileUtils.remove_entry dir
   #  end
   #
-  def Dir.mktmpdir(prefix_suffix="d", *rest)
-    path = Tmpname.create(prefix_suffix, *rest) {|n| mkdir(n, 0700)}
+  def Dir.mktmpdir(prefix_suffix=nil, *rest)
+    path = Tmpname.create(prefix_suffix || "d", *rest) {|n| mkdir(n, 0700)}
     if block_given?
       begin
         yield path