diff --git a/src/pylast/__init__.py b/src/pylast/__init__.py
index d510129..fd7b3e7 100644
--- a/src/pylast/__init__.py
+++ b/src/pylast/__init__.py
@@ -785,7 +785,7 @@ class _ShelfCacheBackend:
     """Used as a backend for caching cacheable requests."""
 
     def __init__(self, file_path=None, flag=None):
-        if flag:
+        if flag is not None:
             self.shelf = shelve.open(file_path, flag=flag)
         else:
             self.shelf = shelve.open(file_path)