diff --git a/fastNLP/core/sampler.py b/fastNLP/core/sampler.py
index 67ec2a8d..4a523f10 100644
--- a/fastNLP/core/sampler.py
+++ b/fastNLP/core/sampler.py
@@ -73,6 +73,7 @@ class BucketSampler(BaseSampler):
         total_sample_num = len(seq_lens)
 
         bucket_indexes = []
+        assert total_sample_num>=self.num_buckets, "The number of samples is smaller than the number of buckets."
         num_sample_per_bucket = total_sample_num // self.num_buckets
         for i in range(self.num_buckets):
             bucket_indexes.append([num_sample_per_bucket * i, num_sample_per_bucket * (i + 1)])