diff --git a/Lib/test/test_import/__init__.py b/Lib/test/test_import/__init__.py
index b09065f812c..f9e8558d1a7 100644
--- a/Lib/test/test_import/__init__.py
+++ b/Lib/test/test_import/__init__.py
@@ -2157,6 +2157,8 @@ class SubinterpImportTests(unittest.TestCase):
             self.check_incompatible_here(module)
         with self.subTest(f'{module}: strict, fresh'):
             self.check_incompatible_fresh(module)
+        with self.subTest(f'{module}: isolated, fresh'):
+            self.check_incompatible_fresh(module, isolated=True)
 
     @unittest.skipIf(_testmultiphase is None, "test requires _testmultiphase module")
     def test_multi_init_extension_compat(self):
diff --git a/Python/import.c b/Python/import.c
index 10ac49fd95f..8cf97b51f65 100644
--- a/Python/import.c
+++ b/Python/import.c
@@ -1969,7 +1969,17 @@ import_run_extension(PyThreadState *tstate, PyModInitFunction p0,
             if (info->filename != NULL) {
                 // XXX There's a refleak somewhere with the filename.
                 // Until we can track it down, we intern it.
-                PyObject *filename = Py_NewRef(info->filename);
+                PyObject *filename = NULL;
+                if (switched) {
+                    // The original filename may be allocated by subinterpreter's
+                    // obmalloc, so we create a copy here.
+                    filename = _PyUnicode_Copy(info->filename);
+                    if (filename == NULL) {
+                        return NULL;
+                    }
+                } else {
+                    filename = Py_NewRef(info->filename);
+                }
                 PyUnicode_InternInPlace(&filename);
                 if (PyModule_AddObjectRef(mod, "__file__", filename) < 0) {
                     PyErr_Clear(); /* Not important enough to report */