...

src/calibre/gui2/device.py

@@ -1771,7 +1771,10 @@ class DeviceMixin(object):  # {{{
         string_pat = re.compile(r'(?u)\W|[_]')
 
         def clean_string(x):
+            try:
                 x = x.lower() if x else ''
+            except Exception:
+                x = ''
             return string_pat.sub('', x)
 
         update_metadata = (