diff --git a/Lib/test/test_dict.py b/Lib/test/test_dict.py
index 2e6c2bbdf19..665b3e843dd 100644
--- a/Lib/test/test_dict.py
+++ b/Lib/test/test_dict.py
@@ -1621,6 +1621,14 @@ def __eq__(self, other):
 
         self.assertEqual(len(d), 1)
 
+    def test_split_table_update_with_str_subclass(self):
+        class MyStr(str): pass
+        class MyClass: pass
+        obj = MyClass()
+        obj.attr = 1
+        obj.__dict__[MyStr('attr')] = 2
+        self.assertEqual(obj.attr, 2)
+
 
 class CAPITest(unittest.TestCase):
 
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-12-03-11-03-35.gh-issue-142218.44Fq_J.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-12-03-11-03-35.gh-issue-142218.44Fq_J.rst
new file mode 100644
index 00000000000..a8ce0fc6526
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-12-03-11-03-35.gh-issue-142218.44Fq_J.rst
@@ -0,0 +1,2 @@
+Fix crash when inserting into a split table dictionary with a non
+:class:`str` key that matches an existing key.
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index ee1c173ae4a..e0eef7b46df 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -1914,10 +1914,14 @@ insertdict(PyInterpreterState *interp, PyDictObject *mp,
     if (old_value != value) {
         _PyDict_NotifyEvent(interp, PyDict_EVENT_MODIFIED, mp, key, value);
         assert(old_value != NULL);
-        assert(!_PyDict_HasSplitTable(mp));
         if (DK_IS_UNICODE(mp->ma_keys)) {
-            PyDictUnicodeEntry *ep = &DK_UNICODE_ENTRIES(mp->ma_keys)[ix];
-            STORE_VALUE(ep, value);
+            if (_PyDict_HasSplitTable(mp)) {
+                STORE_SPLIT_VALUE(mp, ix, value);
+            }
+            else {
+                PyDictUnicodeEntry *ep = &DK_UNICODE_ENTRIES(mp->ma_keys)[ix];
+                STORE_VALUE(ep, value);
+            }
         }
         else {
             PyDictKeyEntry *ep = &DK_ENTRIES(mp->ma_keys)[ix];