diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -138,9 +138,6 @@ def filemerge(repo, mynode, orig, fcd, f
     if not fco.cmp(fcd.data()): # files identical?
         return None
 
-    if fca == fco: # backwards, use working dir parent as ancestor
-        fca = fcd.parents()[0]
-
     ui = repo.ui
     fd = fcd.path()
     binary = isbin(fcd) or isbin(fco) or isbin(fca)
diff --git a/mercurial/merge.py b/mercurial/merge.py
--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -270,7 +270,12 @@ def applyupdates(repo, action, wctx, mct
             repo.ui.debug("preserving %s for resolve of %s\n" % (f, fd))
             fcl = wctx[f]
             fco = mctx[f2]
-            fca = fcl.ancestor(fco, actx) or repo.filectx(f, fileid=nullrev)
+            if mctx == actx: # backwards, use working dir parent as ancestor
+                fca = fcl.parents()[0]
+            else:
+                fca = fcl.ancestor(fco, actx)
+            if not fca:
+                fca = repo.filectx(f, fileid=nullrev)
             ms.add(fcl, fco, fca, fd, flags)
             if f != fd and move:
                 moves.append(f)