diff --git a/mercurial/help.py b/mercurial/help.py
--- a/mercurial/help.py
+++ b/mercurial/help.py
@@ -686,9 +686,13 @@
         # Look for sub-topic entry first.
         header, doc = None, None
         if subtopic and name in subtopics:
+            exists = False
             for names, header, doc in subtopics[name]:
                 if subtopic in names:
+                    exists = True
                     break
+            if not exists:
+                raise error.UnknownCommand(name)
 
         if not header:
             for topic in helptable: