diff --git a/treemanifest/__init__.py b/treemanifest/__init__.py
--- a/treemanifest/__init__.py
+++ b/treemanifest/__init__.py
@@ -510,7 +510,7 @@
 
     def write(self, tr, link, p1, p2, added, removed):
         return _writeclientmanifest(
-            self._treemanifest, tr, self._manifestlog, p1, p2)
+            self._treemanifest, tr, self._manifestlog, None, p1, p2)
 
 def serverreposetup(repo):
     extensions.wrapfunction(manifest.manifestrevlog, 'addgroup',
@@ -599,7 +599,7 @@
 
     return n
 
-def _writeclientmanifest(newtree, tr, tmfl, p1node, p2node):
+def _writeclientmanifest(newtree, tr, tmfl, node, p1node, p2node):
     if not util.safehasattr(tr, 'treedatapack'):
         opener = tmfl._opener
         ui = tmfl.ui
@@ -630,13 +630,18 @@
         tr.addabort('treepack', abort)
         tr.addpending('treepack', writepending)
 
-    dpack = tr.treedatapack
-    hpack = tr.treehistpack
+    # If the manifest was already committed as a flat manifest, use
+    # its node.
+    if node is not None:
+        dpack = InterceptedMutableDataPack(tr.treedatapack, node, p1node)
+        hpack = InterceptedMutableHistoryPack(tr.treehistpack, node, p1node)
+    else:
+        dpack = tr.treedatapack
+        hpack = tr.treehistpack
 
     p1tree = tmfl[p1node].read()
     newtreeiter = newtree.finalize(p1tree)
 
-    node = None
     for nname, nnode, ntext, np1text, np1, np2 in newtreeiter:
         # Not using deltas, since there aren't any other trees in
         # this pack it could delta against.