diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -1287,12 +1287,12 @@ def heads(ui, repo, **opts):
     are the usual targets for update and merge operations.
     """
     if opts['rev']:
-        heads = repo.heads(repo.lookup(rev))
+        heads = repo.heads(repo.lookup(opts['rev']))
     else:
         heads = repo.heads()
     br = None
     if opts['branches']:
-        br = repo.branchlookup(list(heads))
+        br = repo.branchlookup(heads)
     for n in heads:
         show_changeset(ui, repo, changenode=n, brinfo=br)
 
@@ -2241,7 +2241,7 @@ table = {
     "heads":
         (heads,
          [('b', 'branches', None, _('find branch info')),
-          ('r', 'rev', None, _('show only heads descendants from rev'))],
+          ('r', 'rev', "", _('show only heads which are descendants of rev'))],
          _('hg heads [-b] [-r <rev>]')),
     "help": (help_, [], _('hg help [COMMAND]')),
     "identify|id": (identify, [], _('hg identify')),
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -613,7 +613,7 @@ class localrepository:
                 self.dirstate.update([dest], "a")
             self.dirstate.copy(source, dest)
 
-    def heads(self, start=nullid):
+    def heads(self, start=None):
         heads = self.changelog.heads(start)
         # sort the output in rev descending order
         heads = [(-self.changelog.rev(h), h) for h in heads]
diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -409,10 +409,15 @@ class revlog:
         assert heads
         return (orderedout, roots, heads)
 
-    def heads(self, start=nullid):
+    def heads(self, start=None):
         """return the list of all nodes that have no children
-        if start is specified, only heads that are children of
-        start will be returned"""
+
+        if start is specified, only heads that are descendants of
+        start will be returned
+
+        """
+        if start is None:
+            start = nullid
         reachable = {start: 1}
         heads = {start: 1}
         startrev = self.rev(start)