diff --git a/modules/datamodels/datamodelUam.py b/modules/datamodels/datamodelUam.py
index d8e7906a..6b7cdc06 100644
--- a/modules/datamodels/datamodelUam.py
+++ b/modules/datamodels/datamodelUam.py
@@ -89,6 +89,14 @@ class Mandate(BaseModel):
         json_schema_extra={"frontend_type": "checkbox", "frontend_readonly": True, "frontend_required": False}
     )
 
+    @field_validator('isSystem', mode='before')
+    @classmethod
+    def _coerceIsSystem(cls, v):
+        """Coerce None to False (for existing DB records without isSystem field)."""
+        if v is None:
+            return False
+        return v
+
 
 registerModelLabels(
     "Mandate",