--- src/gmpy2_mpz_inplace.c.orig	2018-04-14 12:27:51.000000000 -0600
+++ src/gmpy2_mpz_inplace.c	2018-06-26 19:50:45.167530839 -0600
@@ -50,6 +50,8 @@ GMPy_MPZ_IAdd_Slot(PyObject *self, PyObj
                 mpz_sub_ui(result->z, MPZ(self), ((PyLongObject*)other)->ob_digit[0]);
                 return (PyObject*)result;
             case 0:
+                mpz_set(result->z, MPZ(self));
+                return (PyObject*)result;
             case 1:
                 mpz_add_ui(result->z, MPZ(self), ((PyLongObject*)other)->ob_digit[0]);
                 return (PyObject*)result;