diff --git a/oml/item/models.py b/oml/item/models.py
index f250fba..0318907 100644
--- a/oml/item/models.py
+++ b/oml/item/models.py
@@ -187,6 +187,8 @@ class Item(db.Model):
                             value = ox.sort_string(value).lower()
                 elif isinstance(value, list): #empty list
                     value = None
+                if not value and sort_type != 'boolean':
+                    value = None
                 if getattr(s, key['id']) != value:
                     setattr(s, key['id'], value)
                     update = True
diff --git a/oml/settings.py b/oml/settings.py
index fefb4d0..4d6f06e 100644
--- a/oml/settings.py
+++ b/oml/settings.py
@@ -101,4 +101,4 @@ USER_AGENT = 'OpenMediaLibrary/%s' % VERSION
 
 DEBUG_HTTP = server.get('debug_http', False)
 
-DB_VERSION = 5
+DB_VERSION = 6
diff --git a/oml/update.py b/oml/update.py
index 0bdc2b2..5d19d76 100644
--- a/oml/update.py
+++ b/oml/update.py
@@ -295,16 +295,14 @@ class Update(Thread):
         db_version = settings.server.get('db_version', 0)
         if db_version < settings.DB_VERSION:
             self.status('Updating...')
-            if db_version < 1:
-                db_version = migrate_1()
-            if db_version < 2:
-                db_version = migrate_2()
             if db_version < 3:
                 db_version = migrate_3()
             if db_version < 4:
                 db_version = migrate_4()
             if db_version < 5:
                 db_version = migrate_5()
+            if db_version < 6:
+                db_version = migrate_6()
             settings.server['db_version'] = settings.DB_VERSION
 
     def run(self):
@@ -317,28 +315,6 @@ class Update(Thread):
         restart_oml()
 
 
-def migrate_1():
-    with db.session() as session:
-        import item.models
-        for s in item.models.Sort.query.filter_by(author=''):
-            s.item.update_sort()
-        session.commit()
-    return 1
-
-def migrate_2():
-    with db.session() as session:
-        import item.models
-        for s in item.models.Sort.query.filter_by(author=''):
-            s.item.update_sort()
-        for s in item.models.Sort.query.filter_by(publisher=''):
-            s.item.update_sort()
-        for s in item.models.Sort.query.filter_by(language=''):
-            s.item.update_sort()
-        for s in item.models.Sort.query.filter_by(place=''):
-            s.item.update_sort()
-        session.commit()
-    return 2
-
 def migrate_3():
     with db.session():
         import item.models
@@ -412,3 +388,22 @@ def migrate_5():
         for i in item.models.Item.query:
             i.sync_metadata()
     return 5
+
+def migrate_6():
+    with db.session() as session:
+        import item.models
+        for s in item.models.Sort.query.filter_by(author=''):
+            s.item.update_sort()
+        for s in item.models.Sort.query.filter_by(publisher=''):
+            s.item.update_sort()
+        for s in item.models.Sort.query.filter_by(language=''):
+            s.item.update_sort()
+        for s in item.models.Sort.query.filter_by(place=''):
+            s.item.update_sort()
+        for s in item.models.Sort.query.filter_by(isbn=''):
+            s.item.update_sort()
+        for s in item.models.Sort.query.filter_by(date=''):
+            s.item.update_sort()
+        session.commit()
+    return 6
+