diff --git a/mercurial/mdiff.py b/mercurial/mdiff.py
--- a/mercurial/mdiff.py
+++ b/mercurial/mdiff.py
@@ -245,6 +245,9 @@ def patch(a, bin):
 def get_matching_blocks(a, b):
     return [(d[0], d[2], d[1] - d[0]) for d in bdiff.blocks(a, b)]
 
+def trivialdiffheader(length):
+    return struct.pack(">lll", 0, 0, length)
+
 patches = mpatch.patches
 patchedsize = mpatch.patchedsize
 textdiff = bdiff.bdiff
diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -1087,9 +1087,13 @@ class revlog(object):
             if infocollect is not None:
                 infocollect(nb)
 
-            d = self.revdiff(a, b)
             p = self.parents(nb)
             meta = nb + p[0] + p[1] + lookup(nb)
+            if a == -1:
+                d = self.revision(nb)
+                meta += mdiff.trivialdiffheader(len(d))
+            else:
+                d = self.revdiff(a, b)
             yield changegroup.genchunk("%s%s" % (meta, d))
 
         yield changegroup.closechunk()