--- a/configmix/variables.py	Sun Apr 25 18:05:26 2021 +0200
+++ b/configmix/variables.py	Mon Apr 26 09:42:42 2021 +0200
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 # :-
-# :Copyright: (c) 2015-2020, Franz Glasner. All rights reserved.
+# :Copyright: (c) 2015-2021, Franz Glasner. All rights reserved.
 # :License:   BSD-3-Clause. See LICENSE.txt for details.
 # :-
 """Variable interpolation: implementation of namespaces and filters
@@ -18,6 +18,7 @@
 from functools import wraps
 
 from .compat import PY2, native_os_str_to_text, u
+from .constants import REF_NAMESPACE
 
 
 _MARKER = object()
@@ -74,7 +75,13 @@
     """Register a new variable namespace `name` and it's implementing
     function `fn`
 
+    ..note:: This function checks that `name` is not the special
+             namespace :data:`~configmix.constants.REF_NAMESPACE`.
+
     """
+    if name == REF_NAMESPACE:
+        raise ValueError(
+            "the special namespace `%s' is not allowed here" % REF_NAMESPACE)
     _varns_registry[name] = fn
 
 
@@ -86,7 +93,13 @@
     :returns: the implementing function
     :exception KeyError: if the namespace `name` doesn't exist
 
+    ..note:: This function checks that `name` is not the special
+             namespace :data:`~configmix.constants.REF_NAMESPACE`.
+
     """
+    if name == REF_NAMESPACE:
+        raise ValueError(
+            "the special namespace `%s' is not allowed here" % REF_NAMESPACE)
     return _varns_registry[_normalized(name)]