diff --git a/eopayment/spplus.py b/eopayment/spplus.py
index e8041e4..e8b1ffc 100644
--- a/eopayment/spplus.py
+++ b/eopayment/spplus.py
@@ -14,14 +14,14 @@ import warnings
 
 import Crypto.Cipher.DES
 from .common import (PaymentCommon, URL, PaymentResponse, RECEIVED, ACCEPTED,
-        PAID, ERROR, ResponseError)
+        PAID, ERROR, ResponseError, force_byte)
 
 def N_(message): return message
 
 __all__ = ['Payment']
 
-KEY_DES_KEY = '\x45\x1f\xba\x4f\x4c\x3f\xd4\x97'
-IV = '\x30\x78\x30\x62\x2c\x30\x78\x30'
+KEY_DES_KEY = b'\x45\x1f\xba\x4f\x4c\x3f\xd4\x97'
+IV = b'\x30\x78\x30\x62\x2c\x30\x78\x30'
 REFERENCE = 'reference'
 ETAT = 'etat'
 SPCHECKOK = 'spcheckok'
@@ -56,7 +56,7 @@ TEST_STATE = ('99',)
 
 
 def decrypt_ntkey(ntkey):
-    key = binascii.unhexlify(ntkey.replace(' ',''))
+    key = binascii.unhexlify(ntkey.replace(b' ', b''))
     return decrypt_key(key)
 
 def decrypt_key(key):
@@ -70,7 +70,7 @@ def extract_values(query_string):
         k, v = kv.split('=', 1)
         if k != 'hmac':
             result.append(v)
-    return ''.join(result)
+    return force_byte(''.join(result))
 
 def sign_ntkey_query(ntkey, query):
     key = decrypt_ntkey(ntkey)
diff --git a/tests/test_spplus.py b/tests/test_spplus.py
index 34f48a1..a1a2058 100644
--- a/tests/test_spplus.py
+++ b/tests/test_spplus.py
@@ -2,8 +2,8 @@ from unittest import TestCase
 import eopayment.spplus as spplus
 
 class SPPlustTest(TestCase):
-    ntkey = '58 6d fc 9c 34 91 9b 86 3f ' \
-        'fd 64 63 c9 13 4a 26 ba 29 74 1e c7 e9 80 79'
+    ntkey = b'58 6d fc 9c 34 91 9b 86 3f ' \
+        b'fd 64 63 c9 13 4a 26 ba 29 74 1e c7 e9 80 79'
 
     tests = [('x=coin', 'c04f8266d6ae3ce37551cce996c751be4a95d10a'),
              ('x=coin&y=toto', 'ef008e02f8dbf5e70e83da416b0b3a345db203de'),