diff --git a/Python/pystate.c b/Python/pystate.c
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -278,24 +278,24 @@ PyState_FindModule(struct PyModuleDef* m
     res = PyList_GET_ITEM(state->modules_by_index, index);
     return res==Py_None ? NULL : res;
 }
 
 int
 _PyState_AddModule(PyObject* module, struct PyModuleDef* def)
 {
     PyInterpreterState *state;
+    if (!def)
+        return -1;
     if (def->m_slots) {
         PyErr_SetString(PyExc_SystemError,
                         "PyState_AddModule called on module with slots");
         return -1;
     }
     state = GET_INTERP_STATE();
-    if (!def)
-        return -1;
     if (!state->modules_by_index) {
         state->modules_by_index = PyList_New(0);
         if (!state->modules_by_index)
             return -1;
     }
     while(PyList_GET_SIZE(state->modules_by_index) <= def->m_base.m_index)
         if (PyList_Append(state->modules_by_index, Py_None) < 0)
             return -1;