diff --git a/spec/integ/matrix-client-methods.spec.ts b/spec/integ/matrix-client-methods.spec.ts
index 8e7211937..5508ceaf0 100644
--- a/spec/integ/matrix-client-methods.spec.ts
+++ b/spec/integ/matrix-client-methods.spec.ts
@@ -173,7 +173,9 @@ describe("MatrixClient", function() {
                 signatures: {},
             };
 
-            httpBackend!.when("POST", inviteSignUrl).respond(200, signature);
+            httpBackend!.when("POST", inviteSignUrl).check(request => {
+                expect(request.queryParams?.mxid).toEqual(client!.getUserId());
+            }).respond(200, signature);
             httpBackend!.when("POST", "/join/" + encodeURIComponent(roomId)).check(request => {
                 expect(request.data.third_party_signed).toEqual(signature);
             }).respond(200, { room_id: roomId });
diff --git a/src/client.ts b/src/client.ts
index c38160c53..732c07316 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -3629,10 +3629,9 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
         let signPromise: Promise<IThirdPartySigned | void> = Promise.resolve();
 
         if (opts.inviteSignUrl) {
-            signPromise = this.http.requestOtherUrl<IThirdPartySigned>(
-                Method.Post,
-                new URL(opts.inviteSignUrl), { mxid: this.credentials.userId },
-            );
+            const url = new URL(opts.inviteSignUrl);
+            url.searchParams.set("mxid", this.credentials.userId!);
+            signPromise = this.http.requestOtherUrl<IThirdPartySigned>(Method.Post, url);
         }
 
         const queryString: Record<string, string | string[]> = {};