--- a/python/mozbuild/mozbuild/configure/__init__.py
+++ b/python/mozbuild/mozbuild/configure/__init__.py
@@ -227,17 +227,18 @@ class ConfigureSandbox(dict):
         return super(ConfigureSandbox, self).__getitem__(key)
 
     def __setitem__(self, key, value):
         if (key in self.BUILTINS or key == '__builtins__' or
                 hasattr(self, '%s_impl' % key)):
             raise KeyError('Cannot reassign builtins')
 
         if (not isinstance(value, DummyFunction) and
-                value not in self._templates):
+                value not in self._templates and
+                not issubclass(value, Exception)):
             raise KeyError('Cannot assign `%s` because it is neither a '
                            '@depends nor a @template' % key)
 
         return super(ConfigureSandbox, self).__setitem__(key, value)
 
     def _resolve(self, arg, need_help_dependency=True):
         if isinstance(arg, DummyFunction):
             assert arg in self._depends