diff --git a/web/src/compose_banner.ts b/web/src/compose_banner.ts
index 7ae49ecda1..aab749230e 100644
--- a/web/src/compose_banner.ts
+++ b/web/src/compose_banner.ts
@@ -1,5 +1,4 @@
 import $ from "jquery";
-import assert from "minimalistic-assert";
 
 import render_compose_banner from "../templates/compose_banner/compose_banner.hbs";
 import render_stream_does_not_exist_error from "../templates/compose_banner/stream_does_not_exist_error.hbs";
@@ -7,6 +6,7 @@ import render_stream_does_not_exist_error from "../templates/compose_banner/stre
 import {$t} from "./i18n";
 import * as scroll_util from "./scroll_util";
 import * as stream_data from "./stream_data";
+import type {StreamSubscription} from "./sub_store";
 
 export let scroll_to_message_banner_message_id: number | null = null;
 export function set_scroll_to_message_banner_message_id(val: number | null): void {
@@ -183,14 +183,11 @@ export function show_stream_does_not_exist_error(stream_name: string): void {
     $("#compose_select_recipient_widget").trigger("click");
 }
 
-export function show_stream_not_subscribed_error(stream_name: string): void {
+export function show_stream_not_subscribed_error(sub: StreamSubscription): void {
     const $banner_container = $("#compose_banners");
     if ($(`#compose_banners .${CSS.escape(CLASSNAMES.user_not_subscribed)}`).length) {
         return;
     }
-    const sub = stream_data.get_sub(stream_name);
-    // We expect this to be a does-not-exist error if it was undefined.
-    assert(sub !== undefined);
     const new_row_html = render_compose_banner({
         banner_type: ERROR,
         banner_text: $t({
diff --git a/web/src/compose_validate.ts b/web/src/compose_validate.ts
index 379e054bb0..ebb279da34 100644
--- a/web/src/compose_validate.ts
+++ b/web/src/compose_validate.ts
@@ -540,11 +540,11 @@ export function validate_stream_message_mentions(opts: StreamWildcardOptions): b
     return true;
 }
 
-export function validate_stream_message_address_info(stream_name: string): boolean {
-    if (stream_data.is_subscribed_by_name(stream_name)) {
+export function validate_stream_message_address_info(sub: StreamSubscription): boolean {
+    if (sub.subscribed) {
         return true;
     }
-    compose_banner.show_stream_not_subscribed_error(stream_name);
+    compose_banner.show_stream_not_subscribed_error(sub);
     return false;
 }
 
@@ -598,7 +598,7 @@ function validate_stream_message(scheduling_message: boolean): boolean {
     );
 
     if (
-        !validate_stream_message_address_info(sub.name) ||
+        !validate_stream_message_address_info(sub) ||
         !validate_stream_message_mentions({
             stream_id: sub.stream_id,
             $banner_container,
diff --git a/web/tests/compose_validate.test.js b/web/tests/compose_validate.test.js
index c56a71593e..2e1688a3eb 100644
--- a/web/tests/compose_validate.test.js
+++ b/web/tests/compose_validate.test.js
@@ -90,7 +90,7 @@ test_ui("validate_stream_message_address_info", ({mock_template}) => {
         subscribed: true,
     };
     stream_data.add_sub(party_sub);
-    assert.ok(compose_validate.validate_stream_message_address_info("social"));
+    assert.ok(compose_validate.validate_stream_message_address_info(party_sub));
 
     party_sub.subscribed = false;
     stream_data.add_sub(party_sub);
@@ -101,14 +101,14 @@ test_ui("validate_stream_message_address_info", ({mock_template}) => {
         user_not_subscribed_rendered = true;
         return html;
     });
-    assert.ok(!compose_validate.validate_stream_message_address_info("social"));
+    assert.ok(!compose_validate.validate_stream_message_address_info(party_sub));
     assert.ok(user_not_subscribed_rendered);
 
     party_sub.name = "Frontend";
     party_sub.stream_id = 102;
     stream_data.add_sub(party_sub);
     user_not_subscribed_rendered = false;
-    assert.ok(!compose_validate.validate_stream_message_address_info("Frontend"));
+    assert.ok(!compose_validate.validate_stream_message_address_info(party_sub));
     assert.ok(user_not_subscribed_rendered);
 });