diff --git a/mercurial/cmdutil.py b/mercurial/cmdutil.py
--- a/mercurial/cmdutil.py
+++ b/mercurial/cmdutil.py
@@ -111,6 +111,15 @@ def loglimit(opts):
         limit = None
     return limit
 
+def revsingle(repo, revspec, default=None):
+    if not revspec:
+        return repo[default]
+
+    l = revrange(repo, [revspec])
+    if len(l) < 1:
+        raise util.Abort("empty revision set")
+    return repo[l[-1]]
+
 def revpair(repo, revs):
     if not revs:
         return repo.dirstate.parents()[0], None
diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -634,7 +634,7 @@ def cat(ui, repo, file1, *pats, **opts):
 
     Returns 0 on success.
     """
-    ctx = repo[opts.get('rev')]
+    ctx = cmdutil.revsingle(repo, opts.get('rev'))
     err = 1
     m = cmdutil.match(repo, (file1,) + pats, opts)
     for abs in ctx.walk(m):