diff --git a/src/calibre/library/caches.py b/src/calibre/library/caches.py
index 7fa26bd9e9..c77b03c112 100644
--- a/src/calibre/library/caches.py
+++ b/src/calibre/library/caches.py
@@ -144,7 +144,7 @@ def _match(query, value, matchkind):
             pass
     return False
 
-def force_to_bool(self, val):
+def force_to_bool(val):
     if isinstance(val, (str, unicode)):
         try:
             val = icu_lower(val)