diff --git a/ruby.c b/ruby.c
index f0af34a9a8..452d0a0e3c 100644
--- a/ruby.c
+++ b/ruby.c
@@ -1744,6 +1744,9 @@ process_options(int argc, char **argv, ruby_cmdline_options_t *opt)
 		path = str_conv_enc(path, uenc, lenc);
 	    }
 #endif
+	    if (!ENCODING_GET(path)) { /* ASCII-8BIT */
+		rb_enc_copy(path, opt->script_name);
+	    }
 	}
 	base_block = toplevel_context(toplevel_binding);
 	iseq = rb_iseq_new_main(ast->root, opt->script_name, path, vm_block_iseq(base_block));
diff --git a/test/ruby/test_rubyoptions.rb b/test/ruby/test_rubyoptions.rb
index c3ec35d345..083dcec027 100644
--- a/test/ruby/test_rubyoptions.rb
+++ b/test/ruby/test_rubyoptions.rb
@@ -952,16 +952,21 @@ class TestRubyOptions < Test::Unit::TestCase
   end
 
   def test___dir__encoding
+    lang = {"LC_ALL"=>ENV["LC_ALL"]||ENV["LANG"]}
     with_tmpchdir do
       testdir = "\u30c6\u30b9\u30c8"
       Dir.mkdir(testdir)
       Dir.chdir(testdir) do
         open("test.rb", "w") do |f|
           f.puts <<-END
-            p __FILE__.encoding == __dir__.encoding
+            if __FILE__.encoding == __dir__.encoding
+              p true
+            else
+              puts "__FILE__: \#{__FILE__.encoding}, __dir__: \#{__dir__.encoding}"
+            end
           END
         end
-        r, = EnvUtil.invoke_ruby("test.rb", "", true)
+        r, = EnvUtil.invoke_ruby([lang, "test.rb"], "", true)
         assert_equal "true", r.chomp, "the encoding of __FILE__ and __dir__ should be same"
       end
     end