diff --git a/mercurial/help.py b/mercurial/help.py
--- a/mercurial/help.py
+++ b/mercurial/help.py
@@ -689,6 +689,8 @@
             for names, header, doc in subtopics[name]:
                 if subtopic in names:
                     break
+            if not any(subtopic in s[0] for s in subtopics[name]):
+                raise error.UnknownCommand(name)
 
         if not header:
             for topic in helptable: