diff --git a/src/llamafactory/data/loader.py b/src/llamafactory/data/loader.py
index fa5b12c5..d4a19e27 100644
--- a/src/llamafactory/data/loader.py
+++ b/src/llamafactory/data/loader.py
@@ -115,6 +115,7 @@ def load_single_dataset(
             expand_indexes = np.random.choice(len(dataset), target_num)
             indexes = np.concatenate((indexes, expand_indexes), axis=0)
 
+        assert len(indexes) == dataset_attr.num_samples, "Sample num mismatched."
         dataset = dataset.select(indexes)
         logger.info("Sampled {} examples from dataset {}.".format(dataset_attr.num_samples, dataset_attr))