diff --git a/src/calibre/library/prefs.py b/src/calibre/library/prefs.py
index 7fccebbcfe..4514f4cf1e 100644
--- a/src/calibre/library/prefs.py
+++ b/src/calibre/library/prefs.py
@@ -58,13 +58,13 @@ class DBPrefs(dict):
         self.__setitem__(key, val)
 
     def get_namespaced(self, namespace, key, default=None):
-        key = '%s:%s'%(namespace, key)
+        key = u'namespaced:%s:%s'%(namespace, key)
         try:
             return dict.__getitem__(self, key)
         except KeyError:
             return default
 
     def set_namespaced(self, namespace, key, val):
-        key = '%s:%s'%(namespace, key)
+        key = u'namespaced:%s:%s'%(namespace, key)
         self[key] = val