diff --git a/spec/unit/matrix-client.spec.ts b/spec/unit/matrix-client.spec.ts
index c685ea3a2..fbe8c67d7 100644
--- a/spec/unit/matrix-client.spec.ts
+++ b/spec/unit/matrix-client.spec.ts
@@ -87,7 +87,7 @@ describe("MatrixClient", function() {
         // }
         // items are popped off when processed and block if no items left.
     ];
-    let acceptKeepalives;
+    let acceptKeepalives: boolean;
     let pendingLookup = null;
     function httpReq(cb, method, path, qp, data, prefix) {
         if (path === KEEP_ALIVE_PATH && acceptKeepalives) {
@@ -127,7 +127,7 @@ describe("MatrixClient", function() {
                 (next.error ? "BAD" : "GOOD") + " response",
             );
             if (next.expectBody) {
-                expect(next.expectBody).toEqual(data);
+                expect(data).toEqual(next.expectBody);
             }
             if (next.expectQueryParams) {
                 Object.keys(next.expectQueryParams).forEach(function(k) {
@@ -777,7 +777,7 @@ describe("MatrixClient", function() {
                 expectBody: content,
             }];
 
-            await client.sendEvent(roomId, EventType.RoomMessage, content, txnId);
+            await client.sendEvent(roomId, EventType.RoomMessage, { ...content }, txnId);
         });
 
         it("overload with null threadId works", async () => {
@@ -790,20 +790,99 @@ describe("MatrixClient", function() {
                 expectBody: content,
             }];
 
-            await client.sendEvent(roomId, null, EventType.RoomMessage, content, txnId);
+            await client.sendEvent(roomId, null, EventType.RoomMessage, { ...content }, txnId);
         });
 
         it("overload with threadId works", async () => {
             const eventId = "$eventId:example.org";
             const txnId = client.makeTxnId();
+            const threadId = "$threadId:server";
             httpLookups = [{
                 method: "PUT",
                 path: `/rooms/${encodeURIComponent(roomId)}/send/m.room.message/${txnId}`,
                 data: { event_id: eventId },
-                expectBody: content,
+                expectBody: {
+                    ...content,
+                    "m.relates_to": {
+                        "event_id": threadId,
+                        "is_falling_back": true,
+                        "rel_type": "m.thread",
+                    },
+                },
             }];
 
-            await client.sendEvent(roomId, "$threadId:server", EventType.RoomMessage, content, txnId);
+            await client.sendEvent(roomId, threadId, EventType.RoomMessage, { ...content }, txnId);
+        });
+
+        it("should add thread relation if threadId is passed and the relation is missing", async () => {
+            const eventId = "$eventId:example.org";
+            const threadId = "$threadId:server";
+            const txnId = client.makeTxnId();
+
+            const room = new Room(roomId, client, userId);
+            store.getRoom.mockReturnValue(room);
+
+            const rootEvent = new MatrixEvent({ event_id: threadId });
+            room.createThread(threadId, rootEvent, [rootEvent], false);
+
+            httpLookups = [{
+                method: "PUT",
+                path: `/rooms/${encodeURIComponent(roomId)}/send/m.room.message/${txnId}`,
+                data: { event_id: eventId },
+                expectBody: {
+                    ...content,
+                    "m.relates_to": {
+                        "m.in_reply_to": {
+                            event_id: threadId,
+                        },
+                        "event_id": threadId,
+                        "is_falling_back": true,
+                        "rel_type": "m.thread",
+                    },
+                },
+            }];
+
+            await client.sendEvent(roomId, threadId, EventType.RoomMessage, { ...content }, txnId);
+        });
+
+        it("should add thread relation if threadId is passed and the relation is missing with reply", async () => {
+            const eventId = "$eventId:example.org";
+            const threadId = "$threadId:server";
+            const txnId = client.makeTxnId();
+
+            const content = {
+                body,
+                "m.relates_to": {
+                    "m.in_reply_to": {
+                        event_id: "$other:event",
+                    },
+                },
+            };
+
+            const room = new Room(roomId, client, userId);
+            store.getRoom.mockReturnValue(room);
+
+            const rootEvent = new MatrixEvent({ event_id: threadId });
+            room.createThread(threadId, rootEvent, [rootEvent], false);
+
+            httpLookups = [{
+                method: "PUT",
+                path: `/rooms/${encodeURIComponent(roomId)}/send/m.room.message/${txnId}`,
+                data: { event_id: eventId },
+                expectBody: {
+                    ...content,
+                    "m.relates_to": {
+                        "m.in_reply_to": {
+                            event_id: "$other:event",
+                        },
+                        "event_id": threadId,
+                        "is_falling_back": false,
+                        "rel_type": "m.thread",
+                    },
+                },
+            }];
+
+            await client.sendEvent(roomId, threadId, EventType.RoomMessage, { ...content }, txnId);
         });
     });
 
diff --git a/src/client.ts b/src/client.ts
index 92d989816..9a2a0e27d 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -3772,17 +3772,20 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
         // If we expect that an event is part of a thread but is missing the relation
         // we need to add it manually, as well as the reply fallback
         if (threadId && !content["m.relates_to"]?.rel_type) {
+            const isReply = !!content["m.relates_to"]?.["m.in_reply_to"];
             content["m.relates_to"] = {
                 ...content["m.relates_to"],
                 "rel_type": THREAD_RELATION_TYPE.name,
                 "event_id": threadId,
+                // Set is_falling_back to true unless this is actually intended to be a reply
+                "is_falling_back": !isReply,
             };
             const thread = this.getRoom(roomId)?.getThread(threadId);
-            if (thread) {
+            if (thread && !isReply) {
                 content["m.relates_to"]["m.in_reply_to"] = {
                     "event_id": thread.lastReply((ev: MatrixEvent) => {
                         return ev.isRelation(THREAD_RELATION_TYPE.name) && !ev.status;
-                    })?.getId(),
+                    })?.getId() ?? threadId,
                 };
             }
         }
@@ -4031,7 +4034,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
             $txnId: txnId,
         };
 
-        let path;
+        let path: string;
 
         if (event.isState()) {
             let pathTemplate = "/rooms/$roomId/state/$eventType";
diff --git a/src/content-helpers.ts b/src/content-helpers.ts
index 8c813b7aa..8d417d0a9 100644
--- a/src/content-helpers.ts
+++ b/src/content-helpers.ts
@@ -139,10 +139,10 @@ export const getTextForLocationEvent = (
 /**
  * Generates the content for a Location event
  * @param uri a geo:// uri for the location
- * @param ts the timestamp when the location was correct (milliseconds since
+ * @param timestamp the timestamp when the location was correct (milliseconds since
  *           the UNIX epoch)
  * @param description the (optional) label for this location on the map
- * @param asset_type the (optional) asset type of this location e.g. "m.self"
+ * @param assetType the (optional) asset type of this location e.g. "m.self"
  * @param text optional. A text for the location
  */
 export const makeLocationContent = (