diff --git a/src/fiber_package/util.py b/src/fiber_package/util.py
index 57042a7..8973966 100644
--- a/src/fiber_package/util.py
+++ b/src/fiber_package/util.py
@@ -1,23 +1,23 @@
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 
 from pydantic import UUID4
 
 @dataclass(slots=True)
 class EntityList:
     template: str
-    uuids: list[UUID4]
+    uuids: list[UUID4] = field(default_factory=list)
 
 @dataclass(slots=True)
 class GroupList:
     template: str
-    uuids: list[UUID4]
+    uuids: list[UUID4] = field(default_factory=list)
 
 @dataclass(slots=True)
 class EntityRef:
     template: str
-    uuids: UUID4
+    uuids: UUID4 | None = None
 
 @dataclass(slots=True)
 class GroupRef:
     template: str
-    uuids: list[UUID4]
\ No newline at end of file
+    uuids: UUID4 | None = None
\ No newline at end of file