diff --git a/cryptography/primitives/block/ciphers.py b/cryptography/primitives/block/ciphers.py
index 17e7765..3d76afa 100644
--- a/cryptography/primitives/block/ciphers.py
+++ b/cryptography/primitives/block/ciphers.py
@@ -14,6 +14,7 @@
 
 class AES(object):
     name = "AES"
+    block_size = 128
     key_sizes = set([128, 192, 256])
 
     def __init__(self, key):