diff --git a/packages/cultmesh-rs/src/rudp_document_server.rs b/packages/cultmesh-rs/src/rudp_document_server.rs
index 00dc37db..34794fc5 100644
--- a/packages/cultmesh-rs/src/rudp_document_server.rs
+++ b/packages/cultmesh-rs/src/rudp_document_server.rs
@@ -11,6 +11,9 @@ use std::net::{SocketAddr, UdpSocket};
 use std::time::{Duration, Instant, SystemTime, UNIX_EPOCH};
 
 const MAX_UDP_DATAGRAM_BYTES: usize = 65_535;
+/// The shortest message id CultNet encodes: one character. A snapshot request
+/// can carry no shorter id, so no response to it can be smaller.
+const SHORTEST_MESSAGE_ID: &str = "0";
 
 /// The transport identity of one remote CultNet RUDP session.
 ///
@@ -186,7 +189,7 @@ pub enum CultMeshRudpRejectionReason {
     /// The caller's sink refused the document. Carries the sink's error text.
     SinkRefused(String),
     /// The document could never be served: a snapshot response carrying it
-    /// alone, with an empty message id, would exceed
+    /// alone, under the shortest message id CultNet allows, would exceed
     /// `max_snapshot_response_bytes`. It was not offered to the sink.
     DocumentUnservable {
         response_bytes: usize,
@@ -555,9 +558,9 @@ where
                 };
                 // Admit only what some snapshot request can return: the smallest
                 // response that could carry this document is the document alone
-                // under an empty message id, sized by the snapshot path's encoder.
+                // under the shortest message id CultNet encodes, sized by the snapshot path's encoder.
                 let alone = CultNetMessage::SnapshotResponseRaw {
-                    message_id: String::new(),
+                    message_id: SHORTEST_MESSAGE_ID.into(),
                     documents: vec![document],
                 };
                 let response_bytes = match encode_snapshot_response(&alone) {
diff --git a/packages/cultmesh-rs/tests/rudp_document_server.rs b/packages/cultmesh-rs/tests/rudp_document_server.rs
index 2e51cb2d..1d338058 100644
--- a/packages/cultmesh-rs/tests/rudp_document_server.rs
+++ b/packages/cultmesh-rs/tests/rudp_document_server.rs
@@ -212,11 +212,12 @@ fn connect(
 }
 
 /// The encoded size of the snapshot response that carries `document` alone
-/// under an empty message id: the smallest response that could ever serve it.
+/// under a one-character message id, the shortest CultNet encodes: the
+/// smallest response that could ever serve it.
 fn served_alone_bytes(document: &CultNetRawDocumentRecord) -> Result<usize> {
     Ok(encode_cultnet_message_to_vec(
         &CultNetMessage::SnapshotResponseRaw {
-            message_id: String::new(),
+            message_id: "r".into(),
             documents: vec![document.clone()],
         },
         CultNetWireContract::CultNetSchemaV0,
@@ -758,7 +759,7 @@ fn a_put_the_server_could_never_serve_is_refused_and_one_at_the_bound_is_served(
     send(
         &mut writer,
         &CultNetMessage::SnapshotRequest {
-            message_id: String::new(),
+            message_id: "r".into(),
             schema_ids: None,
             record_keys: None,
         },
@@ -781,7 +782,7 @@ fn a_put_the_server_could_never_serve_is_refused_and_one_at_the_bound_is_served(
     assert_eq!(
         decode_cultnet_message_from_slice(&served, CultNetWireContract::CultNetSchemaV0)?,
         CultNetMessage::SnapshotResponseRaw {
-            message_id: String::new(),
+            message_id: "r".into(),
             documents: vec![at_bound],
         }
     );