diff --git a/Lib/test/test_capi/test_bytes.py b/Lib/test/test_capi/test_bytes.py
index c591e986bbce35..d20e5016f969c2 100644
--- a/Lib/test/test_capi/test_bytes.py
+++ b/Lib/test/test_capi/test_bytes.py
@@ -315,10 +315,16 @@ def test_create(self):
         self.assertEqual(writer.get_size(), 3)
         self.assertEqual(writer.finish(), self.result_type(b'abc'))
 
+    def test_finish_with_size(self):
+        # Test PyBytesWriter_FinishWithSize()
         writer = self.create_writer(10, b'abc')
         self.assertEqual(writer.get_size(), 10)
         self.assertEqual(writer.finish_with_size(3), self.result_type(b'abc'))
 
+        writer = self.create_writer(3, b'abc')
+        with self.assertRaises(SystemError):
+            writer.finish_with_size(-3)
+
     def test_write_bytes(self):
          # Test PyBytesWriter_WriteBytes()
          writer = self.create_writer()