diff --git a/hive-c0re/src/matrix.rs b/hive-c0re/src/matrix.rs
index e973f18e..e0bf1ebf 100644
--- a/hive-c0re/src/matrix.rs
+++ b/hive-c0re/src/matrix.rs
@@ -1182,7 +1182,12 @@ pub async fn invite_user(
     // the persisted hive Space.
     let room_id = match room_override {
         Some(r) if r.starts_with('#') => resolve_room_alias(client, admin_token, r).await?,
-        Some(r) => r.to_owned(),
+        Some(r) if r.starts_with('!') => r.to_owned(),
+        Some(r) => anyhow::bail!(
+            "matrix: --room {r:?} is neither a room id nor an alias; \
+             prefix with '!' for a room id (!abc:server) or '#' for an \
+             alias (#name:server)"
+        ),
         None => std::fs::read_to_string(hive_space_room_id_path())
             .map(|s| s.trim().to_owned())
             .context(