diff --git a/kallithea/controllers/pullrequests.py b/kallithea/controllers/pullrequests.py
--- a/kallithea/controllers/pullrequests.py
+++ b/kallithea/controllers/pullrequests.py
@@ -338,6 +338,8 @@ class PullrequestsController(BaseRepoCon
                                               other_repo.scm_instance, other_rev, # org and other "swapped"
                                               org_repo.scm_instance, org_rev,
                                               )
+        if ancestor_rev is None:
+            ancestor_rev = org_repo.scm_instance.EMPTY_CHANGESET
         revisions = [cs.raw_id for cs in cs_ranges]
 
         # hack: ancestor_rev is not an other_rev but we want to show the