diff --git a/src/calibre/devices/smart_device_app/driver.py b/src/calibre/devices/smart_device_app/driver.py
index 7613cb4837..a22ab91f36 100644
--- a/src/calibre/devices/smart_device_app/driver.py
+++ b/src/calibre/devices/smart_device_app/driver.py
@@ -817,7 +817,7 @@ def _read_metadata_cache(self):
                             break
                         raw = fd.read(int(rec_len))
                         book = json.loads(raw.decode('utf-8'), object_hook=from_json)
-                        key = book.keys()[0]
+                        key = list(book.keys())[0]
                         metadata = self.json_codec.raw_to_book(book[key]['book'],
                                                             SDBook, self.PREFIX)
                         book[key]['book'] = metadata