diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -388,6 +388,7 @@ def bundle(ui, repo, fname, dest=None, *
 
     If no destination repository is specified the destination is assumed
     to have all the nodes specified by one or more --base parameters.
+    To create a bundle containing all changesets, use --base null.
 
     The bundle file can then be transferred using conventional means and
     applied to another repository with the unbundle or pull command.