diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -1700,9 +1700,13 @@ class localrepository(object):
             trp = weakref.proxy(tr)
 
             if ctx.files():
-                m1 = p1.manifest()
-                m2 = p2.manifest()
-                m = m1.copy()
+                m1ctx = p1.manifestctx()
+                m2ctx = p2.manifestctx()
+                mctx = m1ctx.copy()
+
+                m = mctx.read()
+                m1 = m1ctx.read()
+                m2 = m2ctx.read()
 
                 # check in files
                 added = []
@@ -1736,9 +1740,9 @@ class localrepository(object):
                 drop = [f for f in removed if f in m]
                 for f in drop:
                     del m[f]
-                mn = self.manifestlog.add(m, trp, linkrev,
-                                          p1.manifestnode(), p2.manifestnode(),
-                                          added, drop)
+                mn = mctx.write(trp, linkrev,
+                                p1.manifestnode(), p2.manifestnode(),
+                                added, drop)
                 files = changed + removed
             else:
                 mn = p1.manifestnode()
diff --git a/mercurial/manifest.py b/mercurial/manifest.py
--- a/mercurial/manifest.py
+++ b/mercurial/manifest.py
@@ -1320,14 +1320,14 @@ class manifestlog(object):
             mancache[node] = m
         return m
 
-    def add(self, m, transaction, link, p1, p2, added, removed):
-        return self._revlog.add(m, transaction, link, p1, p2, added, removed)
-
 class memmanifestctx(object):
     def __init__(self, repo):
         self._repo = repo
         self._manifestdict = manifestdict()
 
+    def _revlog(self):
+        return self._repo.manifestlog._revlog
+
     def new(self):
         return memmanifestctx(self._repo)
 
@@ -1339,6 +1339,10 @@ class memmanifestctx(object):
     def read(self):
         return self._manifestdict
 
+    def write(self, transaction, link, p1, p2, added, removed):
+        return self._revlog().add(self._manifestdict, transaction, link, p1, p2,
+                                  added, removed)
+
 class manifestctx(object):
     """A class representing a single revision of a manifest, including its
     contents, its parent revs, and its linkrev.
@@ -1430,6 +1434,9 @@ class memtreemanifestctx(object):
         self._dir = dir
         self._treemanifest = treemanifest()
 
+    def _revlog(self):
+        return self._repo.manifestlog._revlog
+
     def new(self, dir=''):
         return memtreemanifestctx(self._repo, dir=dir)
 
@@ -1441,6 +1448,10 @@ class memtreemanifestctx(object):
     def read(self):
         return self._treemanifest
 
+    def write(self, transaction, link, p1, p2, added, removed):
+        return self._revlog().add(self._treemanifest, transaction, link, p1, p2,
+                                  added, removed)
+
 class treemanifestctx(object):
     def __init__(self, repo, dir, node):
         self._repo = repo