diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
index 4b2d0554d44..6f6e96fba07 100644
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -1142,6 +1142,15 @@ def test_raiseMemError(self):
         self.assertRaises(MemoryError, alloc)
         self.assertRaises(MemoryError, alloc)
 
+    def test_format_subclass(self):
+        class U(unicode):
+            def __unicode__(self):
+                return u'__unicode__ overridden'
+        u = U(u'xxx')
+        self.assertEquals("%s" % u, u'__unicode__ overridden')
+        self.assertEquals("{0}".format(u), u'__unicode__ overridden')
+
+
 def test_main():
     test_support.run_unittest(__name__)
 
diff --git a/Misc/NEWS b/Misc/NEWS
index e4500a89796..d4a11dc7348 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,8 @@ What's New in Python 2.6.6 alpha 1?
 Core and Builtins
 -----------------
 
+- Issue #1583863: An unicode subclass can now override the __unicode__ method
+
 - Issue #7544: Preallocate thread memory before creating the thread to avoid
   a fatal error in low memory condition.
 
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 133cae5d5f6..111f9bb096e 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -8653,7 +8653,7 @@ PyObject *PyUnicode_Format(PyObject *format,
 
             case 's':
             case 'r':
-                if (PyUnicode_Check(v) && c == 's') {
+                if (PyUnicode_CheckExact(v) && c == 's') {
                     temp = v;
                     Py_INCREF(temp);
                 }