diff --git a/DjangoBlog/whoosh_cn_backend.py b/DjangoBlog/whoosh_cn_backend.py
index 52e304a..d8b43cc 100644
--- a/DjangoBlog/whoosh_cn_backend.py
+++ b/DjangoBlog/whoosh_cn_backend.py
@@ -624,7 +624,7 @@ class WhooshSearchBackend(BaseSearchBackend):
                     if string_key in index.fields and hasattr(index.fields[string_key], 'convert'):
                         # Special-cased due to the nature of KEYWORD fields.
                         if index.fields[string_key].is_multivalued:
-                            if value is None or len(value) is 0:
+                            if value is None or len(value) == 0:
                                 additional_fields[string_key] = []
                             else:
                                 additional_fields[string_key] = value.split(',')