diff -r 79279550c8ff -r 777e388c06d6 mercurial/mdiff.py
--- a/mercurial/mdiff.py	Wed Jun 08 10:46:39 2005 -0800
+++ b/mercurial/mdiff.py	Wed Jun 08 10:47:10 2005 -0800
@@ -27,6 +27,7 @@
         a = a.splitlines(1)
         b = b.splitlines(1)
         l = list(difflib.unified_diff(a, b, "a/" + fn, "b/" + fn))
+        if not l: return ""
         # difflib uses a space, rather than a tab
         l[0] = l[0][:-2] + "\t" + ad + "\n"
         l[1] = l[1][:-2] + "\t" + bd + "\n"