diff --git a/pykolab/auth/ldap/auth_cache.py b/pykolab/auth/ldap/auth_cache.py
--- a/pykolab/auth/ldap/auth_cache.py
+++ b/pykolab/auth/ldap/auth_cache.py
@@ -17,6 +17,7 @@
 #
 
 import datetime
+import os
 
 import sqlalchemy
 
@@ -84,6 +85,11 @@
 
     db = init_db()
 
+    if db is None:
+        return
+
+    purge_entries()
+
     try:
         db.query(Entry).filter_by(key=key).delete()
     except sqlalchemy.exc.OperationalError:
@@ -102,6 +108,9 @@
 
     db = init_db()
 
+    if db is None:
+        return None
+
     try:
         _entries = db.query(Entry).filter_by(key=key).all()
     except sqlalchemy.exc.OperationalError:
@@ -121,8 +130,14 @@
 
 
 def set_entry(key, value):
+    # pylint: disable=global-statement
+    global db
+
     db = init_db()
 
+    if db is None:
+        return
+
     try:
         _entries = db.query(Entry).filter_by(key=key).all()
     except sqlalchemy.exc.OperationalError:
@@ -147,12 +162,18 @@
         db.commit()
 
 
-def purge_entries(db):
-    db.query(Entry).filter(
-        Entry.last_change <= (datetime.datetime.now() - datetime.timedelta(1))
-    ).delete()
+def purge_entries():
+    # pylint: disable=global-statement
+    global db
 
-    db.commit()
+    try:
+        db.query(Entry).filter(
+            Entry.last_change <= (datetime.datetime.now() - datetime.timedelta(1))
+        ).delete()
+
+        db.commit()
+    except Exception:
+        return
 
 
 def init_db(reinit=False):
@@ -167,11 +188,13 @@
 
     db_uri = conf.get('ldap', 'auth_cache_uri')
 
+    if db_uri == "null":
+        return None
+
     if db_uri is None:
         db_uri = 'sqlite:///%s/auth_cache.db' % (KOLAB_LIB_PATH)
 
         if reinit:
-            import os
             if os.path.isfile('%s/auth_cache.db' % (KOLAB_LIB_PATH)):
                 os.unlink('%s/auth_cache.db' % (KOLAB_LIB_PATH))
 
@@ -181,6 +204,5 @@
 
     Session = sessionmaker(bind=engine)
     db = Session()
-    purge_entries(db)
 
     return db