diff --git a/Include/cpython/longintrepr.h b/Include/cpython/longintrepr.h
index 68dbf9c4382dc5..6d52427508b5fe 100644
--- a/Include/cpython/longintrepr.h
+++ b/Include/cpython/longintrepr.h
@@ -71,6 +71,9 @@ typedef long stwodigits; /* signed variant of twodigits */
0 <= ob_digit[i] <= MASK.
The allocation function takes care of allocating extra memory
so that ob_digit[0] ... ob_digit[abs(ob_size)-1] are actually available.
+ We always allocate memory for at least one digit, so accessing ob_digit[0]
+ is always safe. However, in the case ob_size == 0, the contents of
+ ob_digit[0] may be undefined.
CAUTION: Generic code manipulating subtypes of PyVarObject has to
aware that ints abuse ob_size's sign bit.
diff --git a/Misc/NEWS.d/next/Core and Builtins/2023-01-14-17-03-08.gh-issue-101037.9ATNuf.rst b/Misc/NEWS.d/next/Core and Builtins/2023-01-14-17-03-08.gh-issue-101037.9ATNuf.rst
new file mode 100644
index 00000000000000..a48756657a29d3
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2023-01-14-17-03-08.gh-issue-101037.9ATNuf.rst
@@ -0,0 +1,2 @@
+Fix potential memory underallocation issue for instances of :class:`int`
+subclasses with value zero.
diff --git a/Objects/longobject.c b/Objects/longobject.c
index 1db4ca418e066e..51ac86961c9954 100644
--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -5638,6 +5638,11 @@ long_subtype_new(PyTypeObject *type, PyObject *x, PyObject *obase)
n = Py_SIZE(tmp);
if (n < 0)
n = -n;
+ /* Fast operations for single digit integers (including zero)
+ * assume that there is always at least one digit present. */
+ if (n == 0) {
+ n = 1;
+ }
newobj = (PyLongObject *)type->tp_alloc(type, n);
if (newobj == NULL) {
Py_DECREF(tmp);
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