# HG changeset patch
# User Alexis S. L. Carvalho <alexis@cecm.usp.br>
# Date 1165508111 7200
# Node ID 000d122071b5188617709f2450a9007b8049f19c
# Parent  2631b506e93d8b1b0abff0529cc70866a9efb9c6
fix hg diff -r ''

diff --git a/mercurial/cmdutil.py b/mercurial/cmdutil.py
--- a/mercurial/cmdutil.py
+++ b/mercurial/cmdutil.py
@@ -18,7 +18,7 @@ def revpair(repo, revs):
     be None, meaning use working dir.'''
 
     def revfix(repo, val, defval):
-        if not val and val != 0:
+        if not val and val != 0 and defval is not None:
             val = defval
         return repo.lookup(val)
 
diff --git a/tests/test-diffdir b/tests/test-diffdir
--- a/tests/test-diffdir
+++ b/tests/test-diffdir
@@ -13,3 +13,8 @@ hg diff --nodates -r tip
 
 echo foo > a
 hg diff --nodates
+
+hg diff -r ""
+hg diff -r tip -r ""
+
+true
diff --git a/tests/test-diffdir.out b/tests/test-diffdir.out
--- a/tests/test-diffdir.out
+++ b/tests/test-diffdir.out
@@ -18,3 +18,5 @@ diff -r acd8075edac9 b
 +++ b/b
 @@ -0,0 +1,1 @@
 +123
+abort: Ambiguous identifier!
+abort: Ambiguous identifier!