diff --git a/src/dbi.py b/src/dbi.py
index 7b4e4ca00..92713aa04 100644
--- a/src/dbi.py
+++ b/src/dbi.py
@@ -262,7 +262,8 @@ class FlatfileMapping(MappingInterface):
 
     def vacuum(self):
         infd = file(self.filename)
-        outfd = utils.transactionalFile(self.filename)
+        outfd = utils.transactionalFile(self.filename,
+                                        makeBackupIfSmaller=False)
         outfd.write(infd.readline()) # First line, nextId.
         for line in infd:
             if not line.startswith('-'):