diff --git a/src/calibre/gui2/actions/choose_library.py b/src/calibre/gui2/actions/choose_library.py
index d726241432..001970f9db 100644
--- a/src/calibre/gui2/actions/choose_library.py
+++ b/src/calibre/gui2/actions/choose_library.py
@@ -390,7 +390,7 @@ class ChooseLibraryAction(InterfaceAction):
         #self.dbref = weakref.ref(self.gui.library_view.model().db)
         #self.before_mem = memory()/1024**2
         self.gui.library_moved(loc)
-        #QTimer.singleShot(1000, self.debug_leak)
+        #QTimer.singleShot(5000, self.debug_leak)
 
     def debug_leak(self):
         import gc
@@ -398,7 +398,7 @@ class ChooseLibraryAction(InterfaceAction):
         ref = self.dbref
         for i in xrange(3): gc.collect()
         if ref() is not None:
-            print 11111, ref()
+            print 'DB object alive:', ref()
             for r in gc.get_referrers(ref())[:10]:
                 print r
                 print
diff --git a/src/calibre/gui2/tag_view.py b/src/calibre/gui2/tag_view.py
index 5a57395530..2160e13b65 100644
--- a/src/calibre/gui2/tag_view.py
+++ b/src/calibre/gui2/tag_view.py
@@ -727,7 +727,11 @@ class TagsModel(QAbstractItemModel): # {{{
         for user_cat in sorted(self.db.prefs.get('user_categories', {}).keys(),
                                key=sort_key):
             cat_name = '@' + user_cat # add the '@' to avoid name collision
-            tb_cats.add_user_category(label=cat_name, name=user_cat)
+            try:
+                tb_cats.add_user_category(label=cat_name, name=user_cat)
+            except ValueError:
+                import traceback
+                traceback.print_exc()
         if len(saved_searches().names()):
             tb_cats.add_search_category(label='search', name=_('Searches'))
 
diff --git a/src/calibre/library/caches.py b/src/calibre/library/caches.py
index 7d6511e8a5..af47a79e49 100644
--- a/src/calibre/library/caches.py
+++ b/src/calibre/library/caches.py
@@ -42,7 +42,8 @@ class MetadataBackup(Thread): # {{{
 
     def stop(self):
         self.keep_running = False
-        self.flush()
+
+    def break_cycles(self):
         # Break cycles so that this object doesn't hold references to db
         self.do_write = self.get_metadata_for_dump = self.clear_dirtied = \
             self.set_dirtied = self.db = None
@@ -111,6 +112,8 @@ class MetadataBackup(Thread): # {{{
                     continue
 
             self.in_limbo = None
+        self.flush()
+        self.break_cycles()
 
     def flush(self):
         'Used during shutdown to ensure that a dirtied book is not missed'