diff --git a/BERT/reader/pretraining.py b/BERT/reader/pretraining.py
index 37026df04f43825b1dae6a5066b936e207a2c099..35a3720eaf8d383e19584ed114322685c517ce3a 100644
--- a/BERT/reader/pretraining.py
+++ b/BERT/reader/pretraining.py
@@ -62,6 +62,9 @@ class DataReader(object):
         self.mask_id = self.vocab["[MASK]"]
         self.is_test = is_test
         self.generate_neg_sample = generate_neg_sample
+        if self.in_tokens:
+            assert self.batch_size >= self.max_seq_len, "The number of " \
+                   "tokens in batch should not be smaller than max seq length."
 
         if self.is_test:
             self.epoch = 1