diff --git a/mayan/apps/permissions/classes.py b/mayan/apps/permissions/classes.py
index 032074fe5d..7efd242920 100644
--- a/mayan/apps/permissions/classes.py
+++ b/mayan/apps/permissions/classes.py
@@ -5,6 +5,7 @@ import logging
 from django.core.exceptions import PermissionDenied
 from django.utils.translation import ugettext_lazy as _
 
+from .exceptions import InvalidNamespace
 from .models import StoredPermission
 
 logger = logging.getLogger(__name__)
@@ -19,7 +20,10 @@ class PermissionNamespace(object):
 
     @classmethod
     def get(cls, name):
-        return cls._registry[name]
+        try:
+            return cls._registry[name]
+        except KeyError:
+            raise InvalidNamespace('Invalid namespace name. This is probably an obsolete permission namespace, execute the management command "purge_permissions" and try again.')
 
     def __init__(self, name, label):
         self.name = name
diff --git a/mayan/apps/permissions/exceptions.py b/mayan/apps/permissions/exceptions.py
new file mode 100644
index 0000000000..d55e89a704
--- /dev/null
+++ b/mayan/apps/permissions/exceptions.py
@@ -0,0 +1,9 @@
+from __future__ import unicode_literals
+
+
+class PermissionError(Exception):
+    pass
+
+
+class InvalidNamespace(PermissionError):
+    pass