diff --git a/crates/ruma-federation-api/src/transactions/send_transaction_message/v1.rs b/crates/ruma-federation-api/src/transactions/send_transaction_message/v1.rs
index f6876c3a..456cd51f 100644
--- a/crates/ruma-federation-api/src/transactions/send_transaction_message/v1.rs
+++ b/crates/ruma-federation-api/src/transactions/send_transaction_message/v1.rs
@@ -48,6 +48,8 @@ ruma_api! {
     #[derive(Default)]
     response: {
         /// Map of event IDs and response for each PDU given in the request.
+        // https://github.com/matrix-org/matrix-doc/pull/3618 makes returning `pdus` optional.
+        #[cfg_attr(feature = "unstable-pre-spec", serde(default))]
         #[serde(with = "crate::serde::pdu_process_response")]
         pub pdus: BTreeMap<Box<EventId>, Result<(), String>>,
     }