diff --git a/src/cowrie/output/dshield.py b/src/cowrie/output/dshield.py
index 6ad3a64d..09224a02 100644
--- a/src/cowrie/output/dshield.py
+++ b/src/cowrie/output/dshield.py
@@ -88,7 +88,7 @@ class Output(cowrie.core.output.Output):
         nonce = base64.b64decode(_nonceb64)
         digest = base64.b64encode(
             hmac.new(
-                '{0}{1}'.format(nonce, self.userid),
+                b'{0}{1}'.format(nonce, self.userid),
                 base64.b64decode(self.auth_key),
                 hashlib.sha256).digest()
         )