diff --git a/src/calibre/db/backend.py b/src/calibre/db/backend.py
index baae1e96a5..abe07a12a1 100644
--- a/src/calibre/db/backend.py
+++ b/src/calibre/db/backend.py
@@ -106,10 +106,15 @@ class DBPrefs(dict):  # {{{
         self.db.execute('DELETE FROM preferences WHERE key=?', (key,))
 
     def __setitem__(self, key, val):
-        if not self.disable_setting and val != self[key]:
-            raw = self.to_raw(val)
-            self.db.execute('INSERT OR REPLACE INTO preferences (key,val) VALUES (?,?)', (key, raw))
-            dict.__setitem__(self, key, val)
+        if not self.disable_setting:
+            try:
+                current_val = self[key]
+            except KeyError:
+                current_val = object()
+            if val != current_val:
+                raw = self.to_raw(val)
+                self.db.execute('INSERT OR REPLACE INTO preferences (key,val) VALUES (?,?)', (key, raw))
+                dict.__setitem__(self, key, val)
 
     def set(self, key, val):
         self.__setitem__(key, val)