diff --git a/DuetRRFOutputDevice.py b/DuetRRFOutputDevice.py
index acc5f4d..9be9235 100644
--- a/DuetRRFOutputDevice.py
+++ b/DuetRRFOutputDevice.py
@@ -134,7 +134,7 @@ def _send(self, command, query=None, next_stage=None, data=None, on_error=None,
 
         if self._http_user and self._http_password:
             auth = "{}:{}".format(self._http_user, self._http_password).encode()
-            headers['Authorization'] = 'Basic ' + base64.b64encode(auth)
+            headers['Authorization'] = 'Basic ' + base64.b64encode(auth).decode()
 
         if data:
             headers['Content-Type'] = 'application/octet-stream'