diff --git a/spec/integ/crypto/crypto.spec.ts b/spec/integ/crypto/crypto.spec.ts
index 50b5f4dea05..fd3b2778cfd 100644
--- a/spec/integ/crypto/crypto.spec.ts
+++ b/spec/integ/crypto/crypto.spec.ts
@@ -630,6 +630,27 @@ describe.each(Object.entries(CRYPTO_BACKENDS))("crypto (%s)", (backend: string,
                 expect(ev.decryptionFailureReason).toEqual(DecryptionFailureCode.HISTORICAL_MESSAGE_USER_NOT_JOINED);
             });
 
+            newBackendOnly(
+                "fails with NOT_JOINED if user is not member of room (MSC4115 unstable prefix)",
+                async () => {
+                    fetchMock.get("path:/_matrix/client/v3/room_keys/version", {
+                        status: 404,
+                        body: { errcode: "M_NOT_FOUND", error: "No current backup version." },
+                    });
+                    expectAliceKeyQuery({ device_keys: { "@alice:localhost": {} }, failures: {} });
+                    await startClientAndAwaitFirstSync();
+
+                    const ev = await sendEventAndAwaitDecryption({
+                        unsigned: {
+                            [UNSIGNED_MEMBERSHIP_FIELD.altName!]: "leave",
+                        },
+                    });
+                    expect(ev.decryptionFailureReason).toEqual(
+                        DecryptionFailureCode.HISTORICAL_MESSAGE_USER_NOT_JOINED,
+                    );
+                },
+            );
+
             newBackendOnly(
                 "fails with another error when the server reports user was a member of the room",
                 async () => {
@@ -654,6 +675,30 @@ describe.each(Object.entries(CRYPTO_BACKENDS))("crypto (%s)", (backend: string,
                 },
             );
 
+            newBackendOnly(
+                "fails with another error when the server reports user was a member of the room (MSC4115 unstable prefix)",
+                async () => {
+                    // This tests that when the server reports that the user
+                    // was invited at the time the event was sent, then we
+                    // don't get a HISTORICAL_MESSAGE_USER_NOT_JOINED error,
+                    // and instead get some other error, since the user should
+                    // have gotten the key for the event.
+                    fetchMock.get("path:/_matrix/client/v3/room_keys/version", {
+                        status: 404,
+                        body: { errcode: "M_NOT_FOUND", error: "No current backup version." },
+                    });
+                    expectAliceKeyQuery({ device_keys: { "@alice:localhost": {} }, failures: {} });
+                    await startClientAndAwaitFirstSync();
+
+                    const ev = await sendEventAndAwaitDecryption({
+                        unsigned: {
+                            [UNSIGNED_MEMBERSHIP_FIELD.altName!]: "invite",
+                        },
+                    });
+                    expect(ev.decryptionFailureReason).toEqual(DecryptionFailureCode.HISTORICAL_MESSAGE_NO_KEY_BACKUP);
+                },
+            );
+
             newBackendOnly(
                 "fails with another error when the server reports user was a member of the room",
                 async () => {
@@ -676,6 +721,29 @@ describe.each(Object.entries(CRYPTO_BACKENDS))("crypto (%s)", (backend: string,
                     expect(ev.decryptionFailureReason).toEqual(DecryptionFailureCode.HISTORICAL_MESSAGE_NO_KEY_BACKUP);
                 },
             );
+
+            newBackendOnly(
+                "fails with another error when the server reports user was a member of the room (MSC4115 unstable prefix)",
+                async () => {
+                    // This tests that when the server reports the user's
+                    // membership, and reports that the user was joined, then we
+                    // don't get a HISTORICAL_MESSAGE_USER_NOT_JOINED error, and
+                    // instead get some other error.
+                    fetchMock.get("path:/_matrix/client/v3/room_keys/version", {
+                        status: 404,
+                        body: { errcode: "M_NOT_FOUND", error: "No current backup version." },
+                    });
+                    expectAliceKeyQuery({ device_keys: { "@alice:localhost": {} }, failures: {} });
+                    await startClientAndAwaitFirstSync();
+
+                    const ev = await sendEventAndAwaitDecryption({
+                        unsigned: {
+                            [UNSIGNED_MEMBERSHIP_FIELD.altName!]: "join",
+                        },
+                    });
+                    expect(ev.decryptionFailureReason).toEqual(DecryptionFailureCode.HISTORICAL_MESSAGE_NO_KEY_BACKUP);
+                },
+            );
         });
 
         it("Decryption fails with Unable to decrypt for other errors", async () => {
diff --git a/src/@types/event.ts b/src/@types/event.ts
index 5d8a26c9b23..44ef4c1434c 100644
--- a/src/@types/event.ts
+++ b/src/@types/event.ts
@@ -14,7 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import { UnstableValue } from "../NamespacedValue";
+import { NamespacedValue, UnstableValue } from "../NamespacedValue";
 import {
     PolicyRuleEventContent,
     RoomAvatarEventContent,
@@ -302,7 +302,7 @@ export const UNSIGNED_THREAD_ID_FIELD = new UnstableValue("thread_id", "org.matr
  *
  * @experimental
  */
-export const UNSIGNED_MEMBERSHIP_FIELD = new UnstableValue("membership", "io.element.msc4115.membership");
+export const UNSIGNED_MEMBERSHIP_FIELD = new NamespacedValue("membership", "io.element.msc4115.membership");
 
 /**
  * Mapped type from event type to content type for all specified non-state room events.
diff --git a/src/models/event.ts b/src/models/event.ts
index 7b9365aad50..e78a2f5b233 100644
--- a/src/models/event.ts
+++ b/src/models/event.ts
@@ -77,7 +77,6 @@ export interface IUnsigned {
     "invite_room_state"?: StrippedState[];
     "m.relations"?: Record<RelationType | string, any>; // No common pattern for aggregated relations
     [UNSIGNED_THREAD_ID_FIELD.name]?: string;
-    [UNSIGNED_MEMBERSHIP_FIELD.name]?: Membership | string;
 }
 
 export interface IThreadBundledRelationship {
@@ -717,13 +716,9 @@ export class MatrixEvent extends TypedEventEmitter<MatrixEventEmittedEvents, Mat
      * @returns The user's room membership, or `undefined` if the server does
      *   not report it.
      */
-    public getMembershipAtEvent(): Membership | string | undefined {
+    public getMembershipAtEvent(): Optional<Membership | string> {
         const unsigned = this.getUnsigned();
-        if (typeof unsigned[UNSIGNED_MEMBERSHIP_FIELD.name] === "string") {
-            return unsigned[UNSIGNED_MEMBERSHIP_FIELD.name];
-        } else {
-            return undefined;
-        }
+        return UNSIGNED_MEMBERSHIP_FIELD.findIn<Membership | string>(unsigned);
     }
 
     /**