1 files changed, 9 insertions, 0 deletions
diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index d611a31..50c91ab 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -609,6 +609,15 @@ class PyStringIOTest(MemoryTestMixin, MemorySeekTestMixin,
     UnsupportedOperation = pyio.UnsupportedOperation
     EOF = ""
 
+    def test_lone_surrogates(self):
+        # Issue #20424
+        memio = self.ioclass('\ud800')
+        self.assertEqual(memio.read(), '\ud800')
+
+        memio = self.ioclass()
+        memio.write('\ud800')
+        self.assertEqual(memio.getvalue(), '\ud800')
+
 
 class PyStringIOPickleTest(TextIOTestMixin, unittest.TestCase):
     """Test if pickle restores properly the internal state of StringIO.