diff --git a/shadowsocks/crypto/m2.py b/shadowsocks/crypto/m2.py
index 4c7e148..5ad48a8 100644
--- a/shadowsocks/crypto/m2.py
+++ b/shadowsocks/crypto/m2.py
@@ -33,6 +33,8 @@ try:
     __import__('M2Crypto')
 except ImportError:
     has_m2 = False
+if bytes != str:
+    has_m2 = False
 
 
 def create_cipher(alg, key, iv, op, key_as_bytes=0, d=None, salt=None, i=1,