diff --git a/Lib/test/test__interpreters.py b/Lib/test/test__interpreters.py
index ad3ebbfdff64a7..a32d5d81d2bf2d 100644
--- a/Lib/test/test__interpreters.py
+++ b/Lib/test/test__interpreters.py
@@ -485,6 +485,21 @@ def test_signatures(self):
         msg = r'_interpreters.run_func\(\) argument 3 must be dict, not int'
         with self.assertRaisesRegex(TypeError, msg):
             _interpreters.run_func(self.id, lambda: None, shared=1)
+        # See https://github.com/python/cpython/issues/135855
+        msg = r'_interpreters.set___main___attrs\(\) argument 2 must be dict, not int'
+        with self.assertRaisesRegex(TypeError, msg):
+            _interpreters.set___main___attrs(self.id, 1)
+
+    def test_invalid_shared_none(self):
+        msg = r'must be dict, not None'
+        with self.assertRaisesRegex(TypeError, msg):
+            _interpreters.exec(self.id, 'a', shared=None)
+        with self.assertRaisesRegex(TypeError, msg):
+            _interpreters.run_string(self.id, 'a', shared=None)
+        with self.assertRaisesRegex(TypeError, msg):
+            _interpreters.run_func(self.id, lambda: None, shared=None)
+        with self.assertRaisesRegex(TypeError, msg):
+            _interpreters.set___main___attrs(self.id, None)
 
     def test_invalid_shared_encoding(self):
         # See https://github.com/python/cpython/issues/127196
diff --git a/Misc/NEWS.d/next/Library/2025-06-23-10-19-11.gh-issue-135855.-J0AGF.rst b/Misc/NEWS.d/next/Library/2025-06-23-10-19-11.gh-issue-135855.-J0AGF.rst
new file mode 100644
index 00000000000000..fcf495bdceb168
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-06-23-10-19-11.gh-issue-135855.-J0AGF.rst
@@ -0,0 +1,3 @@
+Raise :exc:`TypeError` instead of :exc:`SystemError` when
+:func:`!_interpreters.set___main___attrs` is passed a non-dict object.
+Patch by Brian Schubert.
diff --git a/Modules/_interpretersmodule.c b/Modules/_interpretersmodule.c
index e201ccd53daf8a..b920c32474f4e6 100644
--- a/Modules/_interpretersmodule.c
+++ b/Modules/_interpretersmodule.c
@@ -1039,8 +1039,8 @@ interp_set___main___attrs(PyObject *self, PyObject *args, PyObject *kwargs)
     PyObject *id, *updates;
     int restricted = 0;
     if (!PyArg_ParseTupleAndKeywords(args, kwargs,
-                                     "OO|$p:" MODULE_NAME_STR ".set___main___attrs",
-                                     kwlist, &id, &updates, &restricted))
+                                     "OO!|$p:" MODULE_NAME_STR ".set___main___attrs",
+                                     kwlist, &id, &PyDict_Type, &updates, &restricted))
     {
         return NULL;
     }
@@ -1054,16 +1054,14 @@ interp_set___main___attrs(PyObject *self, PyObject *args, PyObject *kwargs)
     }
 
     // Check the updates.
-    if (updates != Py_None) {
-        Py_ssize_t size = PyObject_Size(updates);
-        if (size < 0) {
-            return NULL;
-        }
-        if (size == 0) {
-            PyErr_SetString(PyExc_ValueError,
-                            "arg 2 must be a non-empty mapping");
-            return NULL;
-        }
+    Py_ssize_t size = PyDict_Size(updates);
+    if (size < 0) {
+        return NULL;
+    }
+    if (size == 0) {
+        PyErr_SetString(PyExc_ValueError,
+                        "arg 2 must be a non-empty dict");
+        return NULL;
     }
 
     _PyXI_session *session = _PyXI_NewSession();
diff --git a/Python/crossinterp.c b/Python/crossinterp.c
index c44bcd559000b3..16a23f0351cd26 100644
--- a/Python/crossinterp.c
+++ b/Python/crossinterp.c
@@ -2617,6 +2617,7 @@ _PyXI_Enter(_PyXI_session *session,
     // Convert the attrs for cross-interpreter use.
     _PyXI_namespace *sharedns = NULL;
     if (nsupdates != NULL) {
+        assert(PyDict_Check(nsupdates));
         Py_ssize_t len = PyDict_Size(nsupdates);
         if (len < 0) {
             if (result != NULL) {