diff --git a/src/calibre/devices/smart_device_app/driver.py b/src/calibre/devices/smart_device_app/driver.py
index fba450d714..b2ab262323 100644
--- a/src/calibre/devices/smart_device_app/driver.py
+++ b/src/calibre/devices/smart_device_app/driver.py
@@ -1272,7 +1272,7 @@ class SMART_DEVICE_APP(DeviceConfig, DevicePlugin):
                 self._debug('processed cache. count=', len(books_on_device))
                 count_of_cache_items_deleted = 0
                 if self.client_cache_uses_lpaths:
-                    for lpath in self.known_metadata.iterkeys():
+                    for lpath in tuple(self.known_metadata.iterkeys()):
                         if lpath not in lpaths_on_device:
                             try:
                                 uuid = self.known_metadata[lpath].get('uuid', None)
@@ -1569,7 +1569,6 @@ class SMART_DEVICE_APP(DeviceConfig, DevicePlugin):
             traceback.print_exc()
         return None
 
-
     @synchronous('sync_lock')
     def synchronize_with_db(self, db, id_, book):
         from calibre.utils.date import parse_date, is_date_undefined