diff --git a/swarm-secret-client/src/queue.rs b/swarm-secret-client/src/queue.rs
index da1fdb35..ee424e42 100644
--- a/swarm-secret-client/src/queue.rs
+++ b/swarm-secret-client/src/queue.rs
@@ -39,6 +39,12 @@ pub struct Credential {
     pub value: String,
 
     /// The OIDC client id the secret authenticates.
+    ///
+    /// Required, unlike [`crate::matrix::Credential::homeserver`]: that field
+    /// is optional because it was added to objects already in the store, and
+    /// this path is new. An object without an id is not a usable credential,
+    /// so failing to decode is the wanted behaviour — the alternative is a
+    /// reader holding `None` whose only recourse is to spell the id itself.
     pub client_id: String,
 }