diff --git a/test/ruby/test_time.rb b/test/ruby/test_time.rb
index 1f07a82..32d3b97 100644
--- a/test/ruby/test_time.rb
+++ b/test/ruby/test_time.rb
@@ -15,6 +15,22 @@ class TestTime < Test::Unit::TestCase
     $VERBOSE = @verbose
   end
 
+  def test_to_s_default_encoding
+    before = Encoding.default_internal
+    Encoding.default_internal = nil
+    assert_equal Encoding::US_ASCII, Time.now.to_s.encoding
+  ensure
+    Encoding.default_internal = before
+  end
+
+  def test_to_s_transcoding
+    before = Encoding.default_internal
+    Encoding.default_internal = Encoding::UTF_8
+    assert_equal Encoding::UTF_8, Time.now.to_s.encoding
+  ensure
+    Encoding.default_internal = before
+  end
+
   def test_new
     assert_equal(Time.utc(2000,2,10), Time.new(2000,2,10, 11,0,0, 3600*11))
     assert_equal(Time.utc(2000,2,10), Time.new(2000,2,9, 13,0,0, -3600*11))
diff --git a/time.c b/time.c
index 6817c75..a091dcd 100644
--- a/time.c
+++ b/time.c
@@ -4347,6 +4347,8 @@ strftimev(const char *fmt, VALUE time)
     MAKE_TM(time, tobj);
     len = rb_strftime_alloc(&buf, fmt, &tobj->vtm, tobj->timew, TIME_UTC_P(tobj));
     str = rb_str_new(buf, len);
+    rb_enc_associate_index(str, rb_usascii_encindex());
+    str = rb_str_export_to_enc(str, rb_default_internal_encoding());
     if (buf != buffer) xfree(buf);
     return str;
 }