diff --git a/tests/test_pkcs1.py b/tests/test_pkcs1.py
index 39555f6..5702aae 100644
--- a/tests/test_pkcs1.py
+++ b/tests/test_pkcs1.py
@@ -48,7 +48,8 @@ def test_decoding_failure(self):
         a = encrypted[5]
         if is_bytes(a):
             a = ord(a)
-        encrypted = encrypted[:5] + byte(a + 1) + encrypted[6:]
+        altered_a = (a + 1) % 256
+        encrypted = encrypted[:5] + byte(altered_a) + encrypted[6:]
 
         self.assertRaises(pkcs1.DecryptionError, pkcs1.decrypt, encrypted,
                           self.priv)