diff --git a/authentic2_idp_oauth2/views.py b/authentic2_idp_oauth2/views.py
index 4dfdb28..abc1fa0 100644
--- a/authentic2_idp_oauth2/views.py
+++ b/authentic2_idp_oauth2/views.py
@@ -108,7 +108,7 @@ def ws_proxy(request, ws_id):
     logger.debug('proxy to URL %r', url)
     method = request.method.lower()
     if ws.signature_key and ws.auth_mech.startswith('hmac-'):
-        url = signature.sign_url(url, ws.signature_key,
+        url = signature.sign_url(url, str(ws.signature_key),
                 algo=ws.auth_mech[5:])
     response = getattr(requests, method)(url,
             verify=ws.verify_certificate,