--- a/migration.py	Tue Oct 13 12:33:26 2015 +0200
+++ b/migration.py	Mon Sep 21 17:51:58 2015 +0200
@@ -456,6 +456,10 @@
 
 
 def version_strictly_lower(a, b):
+    if a is None:
+        return True
+    if b is None:
+        return False
     if a:
         a = Version(a)
     if b: