diff --git a/mercurial/mdiff.py b/mercurial/mdiff.py
--- a/mercurial/mdiff.py
+++ b/mercurial/mdiff.py
@@ -18,16 +18,22 @@ def unidiff(a, ad, b, bd, fn, r=None, te
 
     if not text and (util.binary(a) or util.binary(b)):
         l = ['Binary file %s has changed\n' % fn]
-    elif a == None:
+    elif not a:
         b = b.splitlines(1)
-        l1 = "--- %s\t%s\n" % ("/dev/null", epoch)
+        if a is None:
+            l1 = "--- %s\t%s\n" % ("/dev/null", epoch)
+        else:
+            l1 = "--- %s\t%s\n" % ("a/" + fn, ad)
         l2 = "+++ %s\t%s\n" % ("b/" + fn, bd)
         l3 = "@@ -0,0 +1,%d @@\n" % len(b)
         l = [l1, l2, l3] + ["+" + e for e in b]
-    elif b == None:
+    elif not b:
         a = a.splitlines(1)
         l1 = "--- %s\t%s\n" % ("a/" + fn, ad)
-        l2 = "+++ %s\t%s\n" % ("/dev/null", epoch)
+        if b is None:
+            l2 = "+++ %s\t%s\n" % ("/dev/null", epoch)
+        else:
+            l2 = "+++ %s\t%s\n" % ("b/" + fn, bd)
         l3 = "@@ -1,%d +0,0 @@\n" % len(a)
         l = [l1, l2, l3] + ["-" + e for e in a]
     else: