diff --git a/Cython/Utility/Optimize.c b/Cython/Utility/Optimize.c
index 5ec6a5fdf26..f2967197c63 100644
--- a/Cython/Utility/Optimize.c
+++ b/Cython/Utility/Optimize.c
@@ -34,7 +34,14 @@ static CYTHON_INLINE int __Pyx_PyList_Append(PyObject* list, PyObject* x) {
     Py_ssize_t len = Py_SIZE(list);
     if (likely(L->allocated > len) & likely(len > (L->allocated >> 1))) {
         Py_INCREF(x);
+        #if CYTHON_COMPILING_IN_CPYTHON && PY_VERSION_HEX >= 0x030d0000
+        // In Py3.13a1, PyList_SET_ITEM() checks that the end index is lower than the current size.
+        // However, extending the size *before* setting the value would not be correct,
+        // so we cannot call PyList_SET_ITEM().
+        L->ob_item[len] = x;
+        #else
         PyList_SET_ITEM(list, len, x);
+        #endif
         __Pyx_SET_SIZE(list, len + 1);
         return 0;
     }
@@ -52,7 +59,14 @@ static CYTHON_INLINE int __Pyx_ListComp_Append(PyObject* list, PyObject* x) {
     Py_ssize_t len = Py_SIZE(list);
     if (likely(L->allocated > len)) {
         Py_INCREF(x);
+        #if CYTHON_COMPILING_IN_CPYTHON && PY_VERSION_HEX >= 0x030d0000
+        // In Py3.13a1, PyList_SET_ITEM() checks that the end index is lower than the current size.
+        // However, extending the size *before* setting the value would not be correct,
+        // so we cannot call PyList_SET_ITEM().
+        L->ob_item[len] = x;
+        #else
         PyList_SET_ITEM(list, len, x);
+        #endif
         __Pyx_SET_SIZE(list, len + 1);
         return 0;
     }