diff --git a/mopidy/frontends/lastfm.py b/mopidy/frontends/lastfm.py
index 52786373..d2c9af88 100644
--- a/mopidy/frontends/lastfm.py
+++ b/mopidy/frontends/lastfm.py
@@ -100,8 +100,8 @@ class LastfmFrontendThread(BaseThread):
                 artists,
                 (track.name or ''),
                 album=(track.album and track.album.name or ''),
-                duration=duration,
-                track_number=track.track_no,
+                duration=str(duration),
+                track_number=str(track.track_no),
                 mbid=(track.musicbrainz_id or ''))
         except (pylast.ScrobblingError, pylast.NetworkError,
                 pylast.MalformedResponseError, pylast.WSError) as e:
@@ -125,10 +125,10 @@ class LastfmFrontendThread(BaseThread):
             self.lastfm.scrobble(
                 artists,
                 (track.name or ''),
-                self.last_start_time,
+                str(self.last_start_time),
                 album=(track.album and track.album.name or ''),
-                track_number=track.track_no,
-                duration=duration,
+                track_number=str(track.track_no),
+                duration=str(duration),
                 mbid=(track.musicbrainz_id or ''))
         except (pylast.ScrobblingError, pylast.NetworkError,
                 pylast.MalformedResponseError, pylast.WSError) as e: