diff --git a/welco/utils.py b/welco/utils.py
index 22629b0..6f92139 100644
--- a/welco/utils.py
+++ b/welco/utils.py
@@ -26,6 +26,7 @@ import requests
 from django.conf import settings
 from django.core.cache import cache
 from django.http import HttpResponse, HttpResponseBadRequest
+from django.utils.encoding import smart_bytes
 from django.utils.http import urlencode, quote
 from django.utils.six.moves.urllib import parse as urlparse
 
@@ -54,7 +55,7 @@ def sign_query(query, key, algo='sha256', timestamp=None, nonce=None):
 
 def sign_string(s, key, algo='sha256', timedelta=30):
     digestmod = getattr(hashlib, algo)
-    hash = hmac.HMAC(str(key), digestmod=digestmod, msg=s)
+    hash = hmac.HMAC(smart_bytes(key), digestmod=digestmod, msg=smart_bytes(s))
     return hash.digest()
 
 def get_wcs_services():