diff --git a/ldapcherry/__init__.py b/ldapcherry/__init__.py
index 642ee2f..d058d80 100644
--- a/ldapcherry/__init__.py
+++ b/ldapcherry/__init__.py
@@ -149,10 +149,10 @@ class LdapCherry(object):
         backends = self.backends_params.keys()
         for b in self.roles.get_backends():
             if b not in backends:
-                raise MissingBackend(b)
-        for b in self.roles.get_backends():
+                raise MissingBackend(b, 'role')
+        for b in self.attributes.get_backends():
             if b not in backends:
-                raise MissingBackend(b)
+                raise MissingBackend(b, 'attribute')
 
     def _init_backends(self, config):
         """ Init all backends
@@ -1003,7 +1003,7 @@ class LdapCherry(object):
     def checkppolicy(self, **params):
         """ search user page """
         self._check_auth(must_admin=False, redir_login=False)
-        keys = params.keys()
+        keys = list(params.keys())
         if len(keys) != 1:
             cherrypy.response.status = 400
             return "bad argument"
diff --git a/ldapcherry/exceptions.py b/ldapcherry/exceptions.py
index 0bd39b5..cb3ac54 100644
--- a/ldapcherry/exceptions.py
+++ b/ldapcherry/exceptions.py
@@ -46,11 +46,12 @@ class MissingRole(Exception):
 
 
 class MissingBackend(Exception):
-    def __init__(self, backend):
+    def __init__(self, backend, type_conf):
         self.backend = backend
         self.log = \
-            "backend '%(backend)s' does not exist in main config file" % \
-            {'backend': backend}
+            "backend '%(backend)s' does not exist in main config file " \
+            "but is still declared in '%(type_conf)s' file" % \
+            {'backend': backend, 'type_conf': type_conf}
 
 
 class WrongBackend(Exception):