--- a/mercurial/mdiff.py
+++ b/mercurial/mdiff.py
@@ -27,6 +27,7 @@ def unidiff(a, ad, b, bd, fn):
         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"