diff -r 8e1b5bbff4c7 Objects/longobject.c
--- a/Objects/longobject.c	Fri May 20 13:37:40 2016 +0200
+++ b/Objects/longobject.c	Sat May 21 18:00:49 2016 +0300
@@ -3022,8 +3022,10 @@
     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) {
+                assert(Py_SIZE(z) != 0);
                 Py_SIZE(z) = -(Py_SIZE(z));
+            }
         }
         else
             z = x_sub(b, a);
@@ -3054,7 +3056,7 @@
             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 {