diff --git a/lib/masterd/instance.py b/lib/masterd/instance.py
index d99f4d87bb5e0d09e8d28b4b8e467cfab047ae9b..095401121518b68c1dad6b01199611a8b03d442d 100644
--- a/lib/masterd/instance.py
+++ b/lib/masterd/instance.py
@@ -1621,7 +1621,7 @@ def ComputeDiskSize(disk_template, disks):
   """
   # Required free disk space as a function of disk and swap space
   req_size_dict = {
-    constants.DT_DISKLESS: None,
+    constants.DT_DISKLESS: 0,
     constants.DT_PLAIN: sum(d[constants.IDISK_SIZE] for d in disks),
     # 128 MB are added for drbd metadata for each disk
     constants.DT_DRBD8: