diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -747,25 +747,35 @@ class revlog(object):
         return c
 
     def lookup(self, id):
-        """locate a node based on revision number or subset of hex nodeid"""
+        """locate a node based on:
+            - revision number or str(revision number)
+            - nodeid or subset of hex nodeid
+        """
         if type(id) == type(0):
+            # rev
             return self.node(id)
         try:
+            # str(rev)
             rev = int(id)
             if str(rev) != id: raise ValueError
             if rev < 0: rev = self.count() + rev
             if rev < 0 or rev >= self.count(): raise ValueError
             return self.node(rev)
         except (ValueError, OverflowError):
-            c = []
-            for n in self.nodemap:
-                if hex(n).startswith(id):
-                    c.append(n)
-            if len(c) > 1: raise RevlogError(_("Ambiguous identifier"))
-            if len(c) == 1: return c[0]
+            pass
+        # hex(node)[:...]
+        node = None
+        for n in self.nodemap:
+            if hex(n).startswith(id):
+                if node is not None:
+                    raise RevlogError(_("Ambiguous identifier"))
+                node = n
+        if node is not None:
+            return node
 
         # might need fixing if we change hash lengths
         if len(id) == 20 and id in self.nodemap:
+            # node
             return id
 
         raise RevlogError(_("No match found"))