--- a/mercurial/mdiff.py	Mon Jan 11 09:49:48 2016 -0800
+++ b/mercurial/mdiff.py	Mon Jan 11 22:00:07 2016 -0500
@@ -376,7 +376,7 @@
     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)
+    return struct.pack(">lll", 0, 0, length) if length else ''
 
 def replacediffheader(oldlen, newlen):
     return struct.pack(">lll", 0, oldlen, newlen)