--- a/diff.py	Fri Nov 10 11:52:37 2017 +0800
+++ b/diff.py	Sun Nov 12 14:22:27 2017 +0800
@@ -109,7 +109,7 @@
                 ]
                 lines += [
                     '  {} {}{}'.format(refmap.get(kind, kind), id, ' ' + role if role is not None else '')
-                    for kind, id, role in versions[version]['refs']
+                    for kind, id, role in versions[version].get('refs', [])
                 ]
     return a, b