diff --git a/hgext/remotefilelog/remotefilelog.py b/hgext/remotefilelog/remotefilelog.py
--- a/hgext/remotefilelog/remotefilelog.py
+++ b/hgext/remotefilelog/remotefilelog.py
@@ -138,7 +138,8 @@
             sidedata = {}
 
         meta, metaoffset = storageutil.parsemeta(text)
-        rawtext, validatehash = self._processflagswrite(text, flags)
+        rawtext, validatehash = self._processflagswrite(text, flags,
+                                                        sidedata=sidedata)
         return self.addrawrevision(rawtext, transaction, linknode, p1, p2,
                                    node, flags, cachedelta,
                                    _metatuple=(meta, metaoffset))
diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -1842,7 +1842,8 @@
         if flags:
             node = node or self.hash(text, p1, p2)
 
-        rawtext, validatehash = self._processflagswrite(text, flags)
+        rawtext, validatehash = self._processflagswrite(text, flags,
+                                                        sidedata=sidedata)
 
         # If the flag processor modifies the revision data, ignore any provided
         # cachedelta.
diff --git a/mercurial/revlogutils/flagutil.py b/mercurial/revlogutils/flagutil.py
--- a/mercurial/revlogutils/flagutil.py
+++ b/mercurial/revlogutils/flagutil.py
@@ -120,7 +120,7 @@
         """
         return self._processflagsfunc(text, flags, 'read')
 
-    def _processflagswrite(self, text, flags):
+    def _processflagswrite(self, text, flags, sidedata):
         """Inspect revision data flags and applies write transformations defined
         by registered flag processors.
 
@@ -136,6 +136,7 @@
         processed text and ``validatehash`` is a bool indicating whether the
         returned text should be checked for hash integrity.
         """
+        assert not sidedata # XXX until it is actually processed
         return self._processflagsfunc(text, flags, 'write')[:2]
 
     def _processflagsraw(self, text, flags):