diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
index acd69df118ab1e81cb5e3fa90cd2b33f2f7aabe4_bWVyY3VyaWFsL2xvY2FscmVwby5weQ==..417f3c27983bcb6ab95b169801edc63c5c138dcf_bWVyY3VyaWFsL2xvY2FscmVwby5weQ== 100644
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -819,8 +819,8 @@
             if not crev:
                 self.ui.debug(" %s: searching for copy revision for %s\n" %
                               (fname, cfname))
-                for ancestor in self['.'].ancestors():
+                for ancestor in self[None].ancestors():
                     if cfname in ancestor:
                         crev = ancestor[cfname].filenode()
                         break
 
@@ -823,11 +823,16 @@
                     if cfname in ancestor:
                         crev = ancestor[cfname].filenode()
                         break
 
-            self.ui.debug(" %s: copy %s:%s\n" % (fname, cfname, hex(crev)))
-            meta["copy"] = cfname
-            meta["copyrev"] = hex(crev)
-            fparent1, fparent2 = nullid, newfparent
+            if crev:
+                self.ui.debug(" %s: copy %s:%s\n" % (fname, cfname, hex(crev)))
+                meta["copy"] = cfname
+                meta["copyrev"] = hex(crev)
+                fparent1, fparent2 = nullid, newfparent
+            else:
+                self.ui.warn(_("warning: can't find ancestor for '%s' "
+                               "copied from '%s'!\n") % (fname, cfname))
+
         elif fparent2 != nullid:
             # is one parent an ancestor of the other?
             fparentancestor = flog.ancestor(fparent1, fparent2)