diff --git a/beetsplug/thumbnails.py b/beetsplug/thumbnails.py
index 086df0f57..5ba6da57f 100644
--- a/beetsplug/thumbnails.py
+++ b/beetsplug/thumbnails.py
@@ -163,7 +163,7 @@ class ThumbnailsPlugin(BeetsPlugin):
         See http://standards.freedesktop.org/thumbnail-spec/latest/x142.html
         """
         metadata = {"Thumb::URI": PurePosixPath(album.artpath).as_uri(),
-                    "Thumb::MTime": os.stat(album.artpath).st_mtime}
+                    "Thumb::MTime": unicode(os.stat(album.artpath).st_mtime)}
         try:
             self.write_metadata(image_path, metadata)
         except Exception:
diff --git a/test/test_thumbnails.py b/test/test_thumbnails.py
index 235b0d88d..8a417ada8 100644
--- a/test/test_thumbnails.py
+++ b/test/test_thumbnails.py
@@ -57,10 +57,12 @@ class ThumbnailsTest(unittest.TestCase, TestHelper):
         plugin = ThumbnailsPlugin()
         plugin.write_metadata = Mock()
         album = Mock(artpath=b"/path/to/cover")
+        mock_stat.return_value.st_mtime = 12345
+
         plugin.add_tags(album, b"/path/to/thumbnail")
 
         metadata = {"Thumb::URI": b"file:///path/to/cover",
-                    "Thumb::MTime": mock_stat.return_value.st_mtime}
+                    "Thumb::MTime": "12345"}
         plugin.write_metadata.assert_called_once_with(b"/path/to/thumbnail",
                                                       metadata)
         mock_stat.assert_called_once_with(album.artpath)