diff --git a/transformers/configuration_utils.py b/transformers/configuration_utils.py
index 8a23be4ff6..228150fc89 100644
--- a/transformers/configuration_utils.py
+++ b/transformers/configuration_utils.py
@@ -56,6 +56,7 @@ class PretrainedConfig(object):
         self.torchscript = kwargs.pop('torchscript', False)
         self.use_bfloat16 = kwargs.pop('use_bfloat16', False)
         self.pruned_heads = kwargs.pop('pruned_heads', {})
+        self.is_decoder = kwargs.pop('is_decoder', False)
 
     def save_pretrained(self, save_directory):
         """ Save a configuration object to the directory `save_directory`, so that it