diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -3837,7 +3837,7 @@ def help_(ui, name=None, **opts):
 
     section = None
     if name and '.' in name:
-        name, section = name.split('.')
+        name, section = name.split('.', 1)
 
     text = help.help_(ui, name, **opts)
 
diff --git a/tests/test-help.t b/tests/test-help.t
--- a/tests/test-help.t
+++ b/tests/test-help.t
@@ -1094,6 +1094,10 @@ Test section lookup
   abort: help section not found
   [255]
 
+  $ hg help glossary.mc.guffin
+  abort: help section not found
+  [255]
+
 Test usage of section marks in help documents
 
   $ cd "$TESTDIR"/../doc