diff --git a/spec/unit/receipt-accumulator.spec.ts b/spec/unit/receipt-accumulator.spec.ts
index 5c9e779de..6e2c71cea 100644
--- a/spec/unit/receipt-accumulator.spec.ts
+++ b/spec/unit/receipt-accumulator.spec.ts
@@ -129,6 +129,50 @@ describe("ReceiptAccumulator", function () {
             ]),
         );
     });
+
+    it("Keeps main thread receipts even when an unthreaded receipt came later", () => {
+        const acc = new ReceiptAccumulator();
+
+        // Given receipts for the special thread "main" and also unthreaded
+        // receipts (which have no thread id).
+        const receipt1 = newReceipt("$event1", ReceiptType.Read, "@alice:localhost", 1, "main");
+        const receipt2 = newReceipt("$event2", ReceiptType.Read, "@alice:localhost", 2);
+
+        // When we collect them
+        acc.consumeEphemeralEvents([receipt1, receipt2]);
+        const newEvent = acc.buildAccumulatedReceiptEvent(roomId);
+
+        // We preserve both: thread:main and unthreaded receipts are different
+        // things, with different meanings.
+        expect(newEvent).toEqual(
+            newMultiReceipt([
+                ["$event1", ReceiptType.Read, "@alice:localhost", 1, "main"],
+                ["$event2", ReceiptType.Read, "@alice:localhost", 2, undefined],
+            ]),
+        );
+    });
+
+    it("Keeps unthreaded receipts even when a main thread receipt came later", () => {
+        const acc = new ReceiptAccumulator();
+
+        // Given receipts for the special thread "main" and also unthreaded
+        // receipts (which have no thread id).
+        const receipt1 = newReceipt("$event1", ReceiptType.Read, "@alice:localhost", 1);
+        const receipt2 = newReceipt("$event2", ReceiptType.Read, "@alice:localhost", 2, "main");
+
+        // When we collect them
+        acc.consumeEphemeralEvents([receipt1, receipt2]);
+        const newEvent = acc.buildAccumulatedReceiptEvent(roomId);
+
+        // We preserve both: thread:main and unthreaded receipts are different
+        // things, with different meanings.
+        expect(newEvent).toEqual(
+            newMultiReceipt([
+                ["$event1", ReceiptType.Read, "@alice:localhost", 1, undefined],
+                ["$event2", ReceiptType.Read, "@alice:localhost", 2, "main"],
+            ]),
+        );
+    });
 });
 
 const newReceipt = (
diff --git a/src/receipt-accumulator.ts b/src/receipt-accumulator.ts
index ce7230f0c..f10372332 100644
--- a/src/receipt-accumulator.ts
+++ b/src/receipt-accumulator.ts
@@ -18,7 +18,7 @@ import { IMinimalEvent } from "./sync-accumulator";
 import { EventType } from "./@types/event";
 import { isSupportedReceiptType, MapWithDefault, recursiveMapToObject } from "./utils";
 import { IContent } from "./models/event";
-import { MAIN_ROOM_TIMELINE, ReceiptContent, ReceiptType } from "./@types/read_receipts";
+import { ReceiptContent, ReceiptType } from "./@types/read_receipts";
 
 interface AccumulatedReceipt {
     data: IMinimalEvent;
@@ -118,7 +118,7 @@ export class ReceiptAccumulator {
                             eventId,
                         };
 
-                        if (!data.thread_id || data.thread_id === MAIN_ROOM_TIMELINE) {
+                        if (!data.thread_id) {
                             this.setUnthreaded(userId, receipt);
                         } else {
                             this.setThreaded(data.thread_id, userId, receipt);