diff --git a/src/models/room-member.ts b/src/models/room-member.ts
index b0a2ba827..fab65ba88 100644
--- a/src/models/room-member.ts
+++ b/src/models/room-member.ts
@@ -25,6 +25,7 @@ import * as utils from "../utils";
 import { User } from "./user";
 import { MatrixEvent } from "./event";
 import { RoomState } from "./room-state";
+import { logger } from "../logger";
 
 export class RoomMember extends EventEmitter {
     private _isOutOfBand = false;
@@ -116,6 +117,15 @@ export class RoomMember extends EventEmitter {
 
         const oldMembership = this.membership;
         this.membership = event.getDirectionalContent().membership;
+        if (this.membership === undefined) {
+            // logging to diagnose https://github.com/vector-im/element-web/issues/20962
+            // (logs event content, although only of membership events)
+            logger.trace(
+                `membership event with membership undefined (forwardLooking: ${event.forwardLooking})!`,
+                event.getContent(),
+                `prevcontent is `, event.getPrevContent(),
+            );
+        }
 
         this.disambiguate = shouldDisambiguate(
             this.userId,