diff --git a/mercurial/mdiff.py b/mercurial/mdiff.py
--- a/mercurial/mdiff.py
+++ b/mercurial/mdiff.py
@@ -160,7 +160,7 @@ def bunidiff(t1, t2, l1, l2, header1, he
         if opts.showfunc:
             # walk backwards from the start of the context
             # to find a line starting with an alphanumeric char.
-            for x in xrange(astart, -1, -1):
+            for x in xrange(astart - 1, -1, -1):
                 t = l1[x].rstrip()
                 if funcre.match(t):
                     func = ' ' + t[:40]
diff --git a/tests/test-diff-unified b/tests/test-diff-unified
--- a/tests/test-diff-unified
+++ b/tests/test-diff-unified
@@ -44,6 +44,15 @@ hg diff --nodates -U 1
 echo '% invalid diff.unified'
 hg --config diff.unified=foo diff --nodates
 
-exit 0
+echo % test off-by-one error with diff -p
+hg init diffp
+cd diffp
+echo a > a
+hg ci -Ama
+rm a
+echo b > a
+echo a >> a
+echo c >> a
+hg diff -U0 -p
 
-
+exit 0