diff --git a/src/calibre/library/sqlite.py b/src/calibre/library/sqlite.py
index 75856dd0f6..83f19b8711 100644
--- a/src/calibre/library/sqlite.py
+++ b/src/calibre/library/sqlite.py
@@ -98,9 +98,10 @@ class AumSortedConcatenate(object):
 
     def finalize(self):
         keys = self.ans.keys()
-        if len(keys) == 0:
-            return None
-        if len(keys) == 1:
+        l = len(keys)
+        if l == 0:
+            return 'Unknown:::Unknown'
+        if l == 1:
             return self.ans[keys[0]]
         return ':#:'.join([self.ans[v] for v in sorted(keys)])