--- a/mercurial/scmutil.py	Sat Nov 07 16:28:30 2020 -0800
+++ b/mercurial/scmutil.py	Tue Dec 01 19:46:01 2020 +0900
@@ -2300,8 +2300,9 @@
 
 
 def bookmarkrevs(repo, mark):
-    """
-    Select revisions reachable by a given bookmark
+    """Select revisions reachable by a given bookmark
+
+    If the bookmarked revision isn't a head, an empty set will be returned.
     """
     return repo.revs(
         b"ancestors(bookmark(%s)) - "