diff --git a/kallithea/controllers/pullrequests.py b/kallithea/controllers/pullrequests.py
--- a/kallithea/controllers/pullrequests.py
+++ b/kallithea/controllers/pullrequests.py
@@ -532,9 +532,14 @@ class PullrequestsController(BaseRepoCon
                             # Note: org_scm_instance.path must come first so all
                             # valid revision numbers are 100% org_scm compatible
                             # - both for avail_revs and for revset results
-                            hgrepo = unionrepo.unionrepository(org_scm_instance.baseui,
-                                                               org_scm_instance.path,
-                                                               other_scm_instance.path)
+                            try:
+                                hgrepo = unionrepo.makeunionrepository(org_scm_instance.baseui,
+                                                                       org_scm_instance.path,
+                                                                       other_scm_instance.path)
+                            except AttributeError: # makeunionrepository was introduced in Mercurial 4.8 23f2299e9e53
+                                hgrepo = unionrepo.unionrepository(org_scm_instance.baseui,
+                                                                   org_scm_instance.path,
+                                                                   other_scm_instance.path)
                         else:
                             hgrepo = org_scm_instance._repo
                         show = set(hgrepo.revs('::%ld & !::parents(%s) & !::%s',