diff --git a/kallithea/controllers/pullrequests.py b/kallithea/controllers/pullrequests.py
--- a/kallithea/controllers/pullrequests.py
+++ b/kallithea/controllers/pullrequests.py
@@ -535,12 +535,7 @@ 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
-                            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,
+                            hgrepo = unionrepo.makeunionrepository(org_scm_instance.baseui,
                                                                    org_scm_instance.path,
                                                                    other_scm_instance.path)
                         else: