diff --git a/paddingoracle.py b/paddingoracle.py
index d772253..5f0e581 100644
--- a/paddingoracle.py
+++ b/paddingoracle.py
@@ -94,7 +94,7 @@ def encrypt(self, plaintext, block_size=8, iv=None, **kwargs):
                                            **kwargs)
 
             block = xor(intermediate_bytes,
-                        plaintext[n - block_size * 2:n + block_size])
+                        plaintext[n - block_size * 2:n - block_size])
 
             encrypted = block + encrypted