diff --git a/src/convert_dataset.py b/src/convert_dataset.py
index 17df9773..e773b76c 100644
--- a/src/convert_dataset.py
+++ b/src/convert_dataset.py
@@ -7,7 +7,7 @@
 import platform
 import warnings
 from argparse import ArgumentParser, Namespace
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from enum import Enum
 from typing import Dict, Iterable, Optional, Union
 
@@ -72,9 +72,9 @@ class DataSplitConstants:
 
 @dataclass
 class DatasetConstants:
-    chars_per_sample: int
-    chars_per_token: int
-    splits = {}
+    chars_per_sample: Optional[int] = None
+    chars_per_token: Optional[int] = None
+    splits: dict[str, DataSplitConstants] = field(default_factory=dict)
 
     def __iter__(self):
         for _, v in self.splits.items():