--- a/urwid_satext/keys.py	Thu Sep 04 17:13:19 2014 +0200
+++ b/urwid_satext/keys.py	Thu Sep 04 18:43:39 2014 +0200
@@ -110,6 +110,9 @@
         assert isinstance(close_namespaces, tuple)
         if always_check is not None:
             assert isinstance(always_check, tuple)
+        to_check = reduce(lambda ns1, ns2: ns1.union(ns2), close_namespaces, set(always_check) or set())
+        if not to_check.issubset(self._namespaces_actions):
+            raise ValueError("Unkown namespaces: {}".format(', '.join(to_check.difference(self._namespaces_actions))))
         self._close_namespaces = close_namespaces
         self._alway_check_namespaces = always_check