diff --git a/activesupport/test/cache/stores/file_store_test.rb b/activesupport/test/cache/stores/file_store_test.rb
index c3c35a7bcc..f6855bb308 100644
--- a/activesupport/test/cache/stores/file_store_test.rb
+++ b/activesupport/test/cache/stores/file_store_test.rb
@@ -68,7 +68,9 @@ def test_key_transformation_with_pathname
   def test_filename_max_size
     key = "#{'A' * ActiveSupport::Cache::FileStore::FILENAME_MAX_SIZE}"
     path = @cache.send(:normalize_key, key, {})
-    Dir::Tmpname.create(path) do |tmpname, n, opts|
+    basename = File.basename(path)
+    dirname = File.dirname(path)
+    Dir::Tmpname.create(basename, Dir.tmpdir + dirname) do |tmpname, n, opts|
       assert File.basename(tmpname + ".lock").length <= 255, "Temp filename too long: #{File.basename(tmpname + '.lock').length}"
     end
   end