diff --git a/src/utils/common.py b/src/utils/common.py
index 917bd867..648f226f 100644
--- a/src/utils/common.py
+++ b/src/utils/common.py
@@ -505,7 +505,7 @@ def preprocess_data(
             input_ids, labels = [], []
 
             for i in range(len(dialog) // 2):
-                source_ids = tokenizer.encode(text=dialog[2*i], add_special_tokens=True)
+                source_ids = tokenizer.encode(text=dialog[2*i], add_special_tokens=(i == 0))
                 target_ids = tokenizer.encode(text=dialog[2*i+1], add_special_tokens=False)
 
                 if len(source_ids) > data_args.max_source_length: