diff -r eaee5aed6fbc Objects/longobject.c
--- a/Objects/longobject.c	Thu May 26 12:19:42 2016 -0700
+++ b/Objects/longobject.c	Sat May 28 19:39:41 2016 +0300
@@ -3022,8 +3022,14 @@
     if (Py_SIZE(a) < 0) {
         if (Py_SIZE(b) < 0) {
             z = x_add(a, b);
-            if (z != NULL && Py_SIZE(z) != 0)
+            if (z != NULL) {
+                /* x_add received two multiple-digit ints, so z must
+                   be a multiple-digit int.
+                   That also means that z is not an element of 
+                   small_ints, so negating it in-place is safe. */
+                assert(Py_ABS(Py_SIZE(z)) > 1);
                 Py_SIZE(z) = -(Py_SIZE(z));
+            }
         }
         else
             z = x_sub(b, a);
@@ -3054,7 +3060,7 @@
             z = x_sub(a, b);
         else
             z = x_add(a, b);
-        if (z != NULL && Py_SIZE(z) != 0)
+        if (z != NULL)
             Py_SIZE(z) = -(Py_SIZE(z));
     }
     else {