diff --git a/Python/pystate.c b/Python/pystate.c
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -249,21 +249,24 @@ PyState_FindModule(struct PyModuleDef* m
 {
     Py_ssize_t index = module->m_base.m_index;
     PyInterpreterState *state = PyThreadState_GET()->interp;
-    PyObject *res;
+    PyObject *wr;
     if (index == 0)
         return NULL;
     if (state->modules_by_index == NULL)
         return NULL;
     if (index >= PyList_GET_SIZE(state->modules_by_index))
         return NULL;
-    res = PyList_GET_ITEM(state->modules_by_index, index);
-    return res==Py_None ? NULL : res;
+    wr = PyList_GET_ITEM(state->modules_by_index, index);
+    if (wr == Py_None)
+        return NULL;
+    return PyWeakref_GET_OBJECT(wr);
 }
 
 int
 _PyState_AddModule(PyObject* module, struct PyModuleDef* def)
 {
     PyInterpreterState *state = PyThreadState_GET()->interp;
+    PyObject *wr;
     if (!def)
         return -1;
     if (!state->modules_by_index) {
@@ -274,9 +277,9 @@ int
     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;
-    Py_INCREF(module);
+    wr = PyWeakref_NewRef(module, NULL);
     return PyList_SetItem(state->modules_by_index,
-                          def->m_base.m_index, module);
+                          def->m_base.m_index, wr);
 }
 
 int
@@ -290,8 +293,9 @@ PyState_AddModule(PyObject* module, stru
     }
     index = def->m_base.m_index;
     if (state->modules_by_index) {
-        if(PyList_GET_SIZE(state->modules_by_index) >= index) {
-            if(module == PyList_GET_ITEM(state->modules_by_index, index)) {
+        if (PyList_GET_SIZE(state->modules_by_index) >= index) {
+            PyObject *wr = PyList_GET_ITEM(state->modules_by_index, index);
+            if (wr != Py_None && module == PyWeakref_GET_OBJECT(wr)) {
                 Py_FatalError("PyState_AddModule: Module already added!");
                 return -1;
             }