diff --git a/src/govoplan_core/core/postbox.py b/src/govoplan_core/core/postbox.py
index feca51f..d6416be 100644
--- a/src/govoplan_core/core/postbox.py
+++ b/src/govoplan_core/core/postbox.py
@@ -176,6 +176,7 @@ class PostboxDirectoryEntryRef:
     key_epoch: int = 1
     encryption_vault_id: str | None = None
     protection_policy: Mapping[str, object] = field(default_factory=dict)
+    grouping_policy: Mapping[str, object] = field(default_factory=dict)
     access: PostboxAccessDecisionRef | None = None
     resource_revision: int = 1
     etag: str | None = None