diff --git a/src/oidcmsg/configure.py b/src/oidcmsg/configure.py
index <HASH>..<HASH> 100644
--- a/src/oidcmsg/configure.py
+++ b/src/oidcmsg/configure.py
@@ -105,12 +105,12 @@ class Base(dict):
             return default
 
     def __setattr__(self, key, value):
-        if key in self and self.key:
+        if key in self and self.key is not None:
             raise KeyError('{} has already been set'.format(key))
         super(Base, self).__setitem__(key, value)
 
     def __setitem__(self, key, value):
-        if key in self:
+        if key in self and self.key is not None:
             raise KeyError('{} has already been set'.format(key))
         super(Base, self).__setitem__(key, value)
Allow an attribute to be set to None before being assigning a value.