diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 1be1b71..03cdfef 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -70,10 +70,13 @@
     def read_ops(self, f):
         data = f.read(5)
         self.assertEqual(data, b"hello")
-        f.readinto(data)
+        n = f.readinto(data)
+        self.assertEqual(n, 5)
         self.assertEqual(data, b" worl")
-        f.readinto(data)
-        self.assertEqual(data, b"d\n")
+        n = f.readinto(data)
+        self.assertEqual(n, 2)
+        self.assertEqual(len(data), 5)
+        self.assertEqual(data[:2], b"d\n")
         f.seek(0)
         self.assertEqual(f.read(20), b"hello world\n")
         f.seek(-6, 2)