diff -r 85640f1feced -r 034d6d0efa7d hgext3rd/topic/__init__.py
--- a/hgext3rd/topic/__init__.py	Wed Mar 11 16:11:04 2020 +0700
+++ b/hgext3rd/topic/__init__.py	Thu Mar 19 20:09:18 2020 +0530
@@ -483,18 +483,27 @@
             if self.ui.configbool(b'experimental', b'enforce-single-head'):
                 if util.safehasattr(tr, 'validator'): # hg <= 4.7 (ebbba3ba3f66)
                     origvalidator = tr.validator
+                elif util.safehasattr(tr, '_validator'):
+                    # hg <= 5.3 (36f08ae87ef6)
+                    origvalidator = tr._validator
                 else:
-                    origvalidator = tr._validator
+                    origvalidator = None
+
+                def _validate(tr2):
+                    repo = reporef()
+                    flow.enforcesinglehead(repo, tr2)
 
                 def validator(tr2):
-                    repo = reporef()
-                    flow.enforcesinglehead(repo, tr2)
+                    _validate(tr2)
                     origvalidator(tr2)
 
                 if util.safehasattr(tr, 'validator'): # hg <= 4.7 (ebbba3ba3f66)
                     tr.validator = validator
+                elif util.safehasattr(tr, '_validator'):
+                    # hg <= 5.3 (36f08ae87ef6)
+                    tr._validator = validator
                 else:
-                    tr._validator = validator
+                    tr.addvalidator(b'000-enforce-single-head', _validate)
 
             topicmodeserver = self.ui.config(b'experimental',
                                              b'topic-mode.server', b'ignore')
@@ -502,17 +511,27 @@
             if (topicmodeserver != b'ignore' and ispush):
                 if util.safehasattr(tr, 'validator'): # hg <= 4.7 (ebbba3ba3f66)
                     origvalidator = tr.validator
-                else:
+                elif util.safehasattr(tr, '_validator'):
+                    # hg <= 5.3 (36f08ae87ef6)
                     origvalidator = tr._validator
+                else:
+                    origvalidator = None
 
-                def validator(tr2):
+                def _validate(tr2):
                     repo = reporef()
                     flow.rejectuntopicedchangeset(repo, tr2)
+
+                def validator(tr2):
+                    _validate(tr2)
                     return origvalidator(tr2)
+
                 if util.safehasattr(tr, 'validator'): # hg <= 4.7 (ebbba3ba3f66)
                     tr.validator = validator
+                elif util.safehasattr(tr, '_validator'):
+                    # hg <= 5.3 (36f08ae87ef6)
+                    tr._validator = validator
                 else:
-                    tr._validator = validator
+                    tr.addvalidator(b'000-reject-untopiced', _validate)
 
             elif (self.ui.configbool(b'experimental', b'topic.publish-bare-branch')
                     and (desc.startswith(b'push')
@@ -533,17 +552,27 @@
             if not allow_publish:
                 if util.safehasattr(tr, 'validator'): # hg <= 4.7 (ebbba3ba3f66)
                     origvalidator = tr.validator
-                else:
+                elif util.safehasattr(tr, '_validator'):
+                    # hg <= 5.3 (36f08ae87ef6)
                     origvalidator = tr._validator
+                else:
+                    origvalidator = None
 
-                def validator(tr2):
+                def _validate(tr2):
                     repo = reporef()
                     flow.reject_publish(repo, tr2)
+
+                def validator(tr2):
+                    _validate(tr2)
                     return origvalidator(tr2)
+
                 if util.safehasattr(tr, 'validator'): # hg <= 4.7 (ebbba3ba3f66)
                     tr.validator = validator
+                elif util.safehasattr(tr, '_validator'):
+                    # hg <= 5.3 (36f08ae87ef6)
+                    tr._validator = validator
                 else:
-                    tr._validator = validator
+                    tr.addvalidator(b'000-reject-publish', _validate)
 
             # real transaction start
             ct = self.currenttopic