diff --git a/src/calibre/db/write.py b/src/calibre/db/write.py
index 175b484c91..7dd278fa57 100644
--- a/src/calibre/db/write.py
+++ b/src/calibre/db/write.py
@@ -94,7 +94,7 @@ def adapt_bool(x):
             x = True
         elif x == 'false':
             x = False
-        elif x == 'none':
+        elif x == 'none' or x == '':
             x = None
         else:
             x = bool(int(x))