diff --git a/STACpopulator/implementations/CMIP6_UofT/add_CMIP6.py b/STACpopulator/implementations/CMIP6_UofT/add_CMIP6.py
index eedecc9..6d6fedb 100644
--- a/STACpopulator/implementations/CMIP6_UofT/add_CMIP6.py
+++ b/STACpopulator/implementations/CMIP6_UofT/add_CMIP6.py
@@ -74,7 +74,7 @@ class CMIP6ItemProperties(STACItemProperties, validate_assignment=True):
     physics_index: int
     realization_index: int
     forcing_index: int
-    tracking_id: str
+    tracking_id: str = ""
     version: str = Field("")
     product: str
     license: str