diff --git a/mercurial/hgweb/hgweb_mod.py b/mercurial/hgweb/hgweb_mod.py
--- a/mercurial/hgweb/hgweb_mod.py
+++ b/mercurial/hgweb/hgweb_mod.py
@@ -884,7 +884,13 @@ class hgweb(object):
         req.write(self.filelog(self.filectx(req)))
 
     def do_lookup(self, req):
-        resp = hex(self.repo.lookup(req.form['key'][0])) + "\n"
+        try:
+            r = hex(self.repo.lookup(req.form['key'][0]))
+            success = 1
+        except Exception,inst:
+            r = str(inst)
+            success = 0
+        resp = "%s %s\n" % (success, r)
         req.httphdr("application/mercurial-0.1", length=len(resp))
         req.write(resp)
 
diff --git a/mercurial/httprepo.py b/mercurial/httprepo.py
--- a/mercurial/httprepo.py
+++ b/mercurial/httprepo.py
@@ -262,12 +262,11 @@ class httprepository(remoterepository):
             fp.close()
 
     def lookup(self, key):
-        try:
-            d = self.do_cmd("lookup", key = key).read()
-            return bin(d[:-1])
-        except:
-            self.ui.warn('Not able to look up revision named "%s"\n' % (key,))
-            raise
+        d = self.do_cmd("lookup", key = key).read()
+        success, data = d[:-1].split(' ', 1)
+        if int(success):
+            return bin(data)
+        raise hg.RepoError(data)
 
     def heads(self):
         d = self.do_read("heads")