diff --git a/src/calibre/utils/config.py b/src/calibre/utils/config.py
index 0bfe201337..22934c9b27 100644
--- a/src/calibre/utils/config.py
+++ b/src/calibre/utils/config.py
@@ -203,7 +203,7 @@ def to_json(obj):
                 '__value__': isoformat(obj, as_utc=True)}
     if isinstance(obj, (set, frozenset)):
         return {'__class__': 'set', '__value__': tuple(obj)}
-    if hasattr(obj, 'toBase64'):
+    if hasattr(obj, 'toBase64'):  # QByteArray
         return {'__class__': 'bytearray',
                 '__value__': bytes(obj.toBase64()).decode('ascii')}
     raise TypeError(repr(obj) + ' is not JSON serializable')