diff --git a/lock/types.proto b/lock/types.proto
index edf141e..da811b8 100644
--- a/lock/types.proto
+++ b/lock/types.proto
@@ -9,7 +9,8 @@ import "refs/types.proto";
 
 // Lock objects protects a list of objects from being deleted. The lifetime of a
 // lock object is limited similar to regular objects in
-// `__NEOFS__EXPIRATION_EPOCH` attribute.
+// `__NEOFS__EXPIRATION_EPOCH` attribute. Lock object MUST have expiration epoch.
+// It is impossible to delete a lock object via ObjectService.Delete RPC call.
 message Lock {
   // List of objects to lock. Must not be empty or carry empty IDs.
   // All members must be of the `REGULAR` type.