--- a/mercurial/archival.py	Wed Apr 08 22:42:37 2015 +0900
+++ b/mercurial/archival.py	Wed Apr 08 23:30:02 2015 +0900
@@ -54,11 +54,17 @@
             return kind
     return None
 
+def _rootctx(repo):
+    # repo[0] may be hidden
+    for rev in repo:
+        return repo[rev]
+    return repo['null']
+
 def buildmetadata(ctx):
     '''build content of .hg_archival.txt'''
     repo = ctx.repo()
     base = 'repo: %s\nnode: %s\nbranch: %s\n' % (
-        repo[0].hex(), ctx.hex(), encoding.fromlocal(ctx.branch()))
+        _rootctx(repo).hex(), ctx.hex(), encoding.fromlocal(ctx.branch()))
 
     tags = ''.join('tag: %s\n' % t for t in ctx.tags()
                    if repo.tagtype(t) == 'global')

--- a/tests/test-obsolete.t	Wed Apr 08 22:42:37 2015 +0900
+++ b/tests/test-obsolete.t	Wed Apr 08 23:30:02 2015 +0900
@@ -250,6 +250,30 @@
   o  0:1f0dee641bb7 (public) [ ] add a
   
 
+  $ cd ..
+
+Revision 0 is hidden
+--------------------
+
+  $ hg init rev0hidden
+  $ cd rev0hidden
+
+  $ mkcommit kill0
+  $ hg up -q null
+  $ hg debugobsolete `getid kill0`
+  $ mkcommit a
+  $ mkcommit b
+
+Should pick the first visible revision as "repo" node
+
+  $ hg archive ../archive-null
+  $ cat ../archive-null/.hg_archival.txt
+  repo: 1f0dee641bb7258c56bd60e93edfa2405381c41e
+  node: 7c3bad9141dcb46ff89abf5f61856facd56e476c
+  branch: default
+  latesttag: null
+  latesttagdistance: 2
+  changessincelatesttag: 2
 
 
   $ cd ..