diff --git a/src/invoke_training/_shared/data/datasets/image_caption_jsonl_dataset.py b/src/invoke_training/_shared/data/datasets/image_caption_jsonl_dataset.py
index ec8499f6..41877225 100644
--- a/src/invoke_training/_shared/data/datasets/image_caption_jsonl_dataset.py
+++ b/src/invoke_training/_shared/data/datasets/image_caption_jsonl_dataset.py
@@ -15,7 +15,7 @@
 
 class ImageCaptionExample(BaseModel):
     image_path: str
-    mask_path: str | None
+    mask_path: str | None = None
     caption: str