diff -r 5c0b6af37b21 -r e46b68547017 hgext3rd/topic/__init__.py
--- a/hgext3rd/topic/__init__.py	Mon Aug 28 17:15:08 2017 +0200
+++ b/hgext3rd/topic/__init__.py	Mon Aug 28 17:35:44 2017 +0200
@@ -345,6 +345,8 @@
         raise error.Abort(_("cannot use --current when setting a topic"))
     if current and clear:
         raise error.Abort(_("cannot use --current and --clear"))
+    if clear and topic:
+        raise error.Abort(_("cannot use --clear when setting a topic"))
 
     if list:
         if clear or rev: