diff --git a/pytorch_transformers/tests/tokenization_tests_commons.py b/pytorch_transformers/tests/tokenization_tests_commons.py
index 3da0494ac4..aa6746a758 100644
--- a/pytorch_transformers/tests/tokenization_tests_commons.py
+++ b/pytorch_transformers/tests/tokenization_tests_commons.py
@@ -186,3 +186,15 @@ class CommonTestCases:
 
             for weights_list_2 in weights_lists_2:
                 self.assertListEqual(weights_list, weights_list_2)
+
+        def test_mask_output(self):
+            if sys.version_info <= (3, 0):
+                return
+
+            tokenizer = self.get_tokenizer()
+
+            if tokenizer.add_special_tokens_sentences_pair.__qualname__.split('.')[0] != "PreTrainedTokenizer":
+                seq_0 = "Test this method."
+                seq_1 = "With these inputs."
+                sequences, mask = tokenizer.encode(seq_0, seq_1, add_special_tokens=True, output_mask=True)
+                assert len(sequences) == len(mask)
diff --git a/pytorch_transformers/tokenization_utils.py b/pytorch_transformers/tokenization_utils.py
index 22ab04ac6a..be49d7eab5 100644
--- a/pytorch_transformers/tokenization_utils.py
+++ b/pytorch_transformers/tokenization_utils.py
@@ -690,6 +690,8 @@ class PreTrainedTokenizer(object):
         if add_special_tokens:
             return self.add_special_tokens_sentences_pair(first_sentence_tokens, second_sentence_tokens, output_mask)
         else:
+            if output_mask:
+                logger.warning("Can't output mask if no special tokens are involved. Please call the method with add_special_tokens set to True.")
             return first_sentence_tokens, second_sentence_tokens
 
     def add_special_tokens_single_sentence(self, token_ids):