diff --git a/lib/tmpdir.rb b/lib/tmpdir.rb
index a5f10fe9cb..967a011cf3 100644
--- a/lib/tmpdir.rb
+++ b/lib/tmpdir.rb
@@ -149,8 +149,8 @@ class Dir
 
     # Generates and yields random names to create a temporary name
     def create(basename, tmpdir=nil, max_try: nil, **opts)
-      origdir = tmpdir
       if tmpdir
+        origdir = tmpdir = File.path(tmpdir)
         raise ArgumentError, "empty parent path" if tmpdir.empty?
       else
         tmpdir = tmpdir()
diff --git a/test/test_tmpdir.rb b/test/test_tmpdir.rb
index eae0610c87..adc29183a8 100644
--- a/test/test_tmpdir.rb
+++ b/test/test_tmpdir.rb
@@ -108,6 +108,16 @@ class TestTmpdir < Test::Unit::TestCase
     assert_raise(ArgumentError) do
       Dir.mktmpdir("foo", "")
     end
+
+    path = Struct.new(:to_path).new("")
+    assert_raise(ArgumentError) do
+      Dir.mktmpdir("foo", path)
+    end
+
+    Dir.mktmpdir do |d|
+      path = Struct.new(:to_path).new(d)
+      assert_operator(Dir.mktmpdir("prefix-", path), :start_with?, d + "/prefix-")
+    end
   end
 
   def assert_mktmpdir_traversal