--- a/hgext3rd/topic/__init__.py	Fri Oct 20 21:43:13 2017 +0200
+++ b/hgext3rd/topic/__init__.py	Fri Oct 20 21:29:14 2017 +0200
@@ -1004,6 +1004,8 @@
             t = opts['topic']
             with repo.vfs.open('topic', 'w') as f:
                 f.write(t)
+        elif opts.get('amend'):
+            pass
         elif notopic and mayabort:
             msg = _("no active topic")
             raise error.Abort(msg, hint=hint)

--- a/tests/test-topic-mode.t	Fri Oct 20 21:43:13 2017 +0200
+++ b/tests/test-topic-mode.t	Fri Oct 20 21:29:14 2017 +0200
@@ -50,15 +50,32 @@
   > EOF
   $ touch a b c d
   $ hg add a
-  $ hg ci -m "Added a"
-  warning: new draft commit without topic
-  (see 'hg help -e topic.topic-mode' for details)
 
 (same test, checking we abort before the editor)
 
-  $ EDITOR=cat hg ci --amend -m "Added a" --edit
+  $ HGEDITOR=cat hg ci -m "Added a" --edit
   warning: new draft commit without topic
   (see 'hg help -e topic.topic-mode' for details)
+  Added a
+  
+  
+  HG: Enter commit message.  Lines beginning with 'HG:' are removed.
+  HG: Leave message empty to abort commit.
+  HG: --
+  HG: user: test
+  HG: branch 'default'
+  HG: added a
+
+  $ HGEDITOR=cat hg ci --amend -m "Added a" --edit
+  Added a
+  
+  
+  HG: Enter commit message.  Lines beginning with 'HG:' are removed.
+  HG: Leave message empty to abort commit.
+  HG: --
+  HG: user: test
+  HG: branch 'default'
+  HG: added a
   $ hg ci --amend -m "added a'" --config experimental.topic-mode=ignore
   $ hg log
   changeset:   2:2e862d8b5eff