diff -urNp Python-2.7.5/Python/bltinmodule.c Python-2.7.5.new/Python/bltinmodule.c
--- Python-2.7.5/Python/bltinmodule.c	2013-05-12 06:32:53.000000000 +0300
+++ Python-2.7.5.new/Python/bltinmodule.c	2013-06-26 07:23:28.000000000 +0300
@@ -2401,32 +2393,43 @@ builtin_sum(PyObject *self, PyObject *ar
     }
 #endif
 
-    for(;;) {
-        item = PyIter_Next(iter);
-        if (item == NULL) {
-            /* error, or end-of-sequence */
-            if (PyErr_Occurred()) {
-                Py_DECREF(result);
-                result = NULL;
-            }
-            break;
-        }
-        /* It's tempting to use PyNumber_InPlaceAdd instead of
-           PyNumber_Add here, to avoid quadratic running time
-           when doing 'sum(list_of_lists, [])'.  However, this
-           would produce a change in behaviour: a snippet like
-
-             empty = []
-             sum([[x] for x in range(10)], empty)
-
-           would change the value of empty. */
-        temp = PyNumber_Add(result, item);
-        Py_DECREF(result);
-        Py_DECREF(item);
-        result = temp;
-        if (result == NULL)
-            break;
-    }
+    /* Add first item separately to make sure we won't change
+       start object in cases like:
+
+         empty = []
+         sum([[x] for x in range(10)], empty)
+
+       */
+    item = PyIter_Next(iter);
+    if (item != NULL) {
+        temp = PyNumber_Add(result, item);
+        Py_DECREF(result);
+        Py_DECREF(item);
+        result = temp;
+        item = PyIter_Next(iter);
+    }
+
+    /* Now add all the other items */
+    while (item != NULL && result != NULL) {
+        temp = PyNumber_InPlaceAdd(result, item);
+        Py_DECREF(result);
+        Py_DECREF(item);
+        result = temp;
+        item = PyIter_Next(iter);
+    }
+
+    /* Cleanup if PyNumber_(InPlace)Add returned NULL */
+    if (item != NULL) {
+        Py_DECREF(item);
+    }
+    else {
+        /* error, or end-of-sequence */
+        if (result != NULL && PyErr_Occurred()) {
+            Py_DECREF(result);
+            result = NULL;
+        }
+    }
+
     Py_DECREF(iter);
     return result;
 }