diff --git a/volia/core/data.py b/volia/core/data.py
index 300b266..d22e8bc 100644
--- a/volia/core/data.py
+++ b/volia/core/data.py
@@ -104,4 +104,7 @@ def write_line(id_, values=[], out=sys.stdout):
         values (list, optional): list of values to write, features or labels. Defaults to [].
         out (_io.TextIOWrapper, optional): . Defaults to sys.stdout.
     """
-    out.write(str(id_) + " ".join(values) + "\n")
\ No newline at end of file
+    if len(values) == 0:
+        out.write(str(id_) + "\n")
+    else:
+        out.write(str(id_) + " " + " ".join(values) + "\n")
\ No newline at end of file