diff --git a/src/@types/matrix-js-sdk.d.ts b/src/@types/matrix-js-sdk.d.ts
new file mode 100644
index 00000000..dc27b1ef
--- /dev/null
+++ b/src/@types/matrix-js-sdk.d.ts
@@ -0,0 +1,18 @@
+/*
+Copyright 2024 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only
+Please see LICENSE in the repository root for full details.
+*/
+
+import {
+  ElementCallReactionEventType,
+  ECallReactionEventContent,
+} from "../reactions";
+
+// Extend Matrix JS SDK types via Typescript declaration merging to support unspecced event fields and types
+declare module "matrix-js-sdk/src/types" {
+  export interface TimelineEvents {
+    [ElementCallReactionEventType]: ECallReactionEventContent;
+  }
+}
diff --git a/src/button/ReactionToggleButton.tsx b/src/button/ReactionToggleButton.tsx
index e5ffb973..f21a88d4 100644
--- a/src/button/ReactionToggleButton.tsx
+++ b/src/button/ReactionToggleButton.tsx
@@ -36,7 +36,6 @@ import { useReactions } from "../useReactions";
 import { useMatrixRTCSessionMemberships } from "../useMatrixRTCSessionMemberships";
 import styles from "./ReactionToggleButton.module.css";
 import {
-  ECallReactionEventContent,
   ReactionOption,
   ReactionSet,
   ElementCallReactionEventType,
@@ -195,7 +194,6 @@ export function ReactionToggleButton({
       const parentEventId = myMembership.eventId;
       try {
         setBusy(true);
-        // @ts-expect-error Trying to send a unspec'd event seems to miss the 3rd overload, need to come back to this.
         await client.sendEvent(
           rtcSession.room.roomId,
           null,
@@ -207,7 +205,8 @@ export function ReactionToggleButton({
             },
             emoji: reaction.emoji,
             name: reaction.name,
-          } as ECallReactionEventContent,
+          },
+          undefined,
         );
         // Do NOT close the menu after this.
       } catch (ex) {