diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -5530,7 +5530,8 @@ def update(ui, repo, node=None, rev=None
 
         # if we defined a bookmark, we have to remember the original name
         brev = rev
-        repo = scmutil.unhidehashlikerevs(repo, [rev], 'nowarn')
+        if rev:
+            repo = scmutil.unhidehashlikerevs(repo, [rev], 'nowarn')
         ctx = scmutil.revsingle(repo, rev, rev)
         rev = ctx.rev()
         if ctx.hidden():
diff --git a/tests/test-directaccess.t b/tests/test-directaccess.t
--- a/tests/test-directaccess.t
+++ b/tests/test-directaccess.t
@@ -162,6 +162,9 @@ This should not throw error
   $ hg up 3
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
 
+  $ hg up
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
 `hg revert`
 
   $ hg revert -r 28ad74 --all