diff --git a/web/src/stream_list.ts b/web/src/stream_list.ts
index 5b55e2e98b..1a990ceb99 100644
--- a/web/src/stream_list.ts
+++ b/web/src/stream_list.ts
@@ -696,7 +696,9 @@ export function refresh_pinned_or_unpinned_stream(sub: StreamSubscription): void
     build_stream_sidebar_row(sub);
     update_streams_sidebar();
     const section_id = stream_list_sort.current_section_id_for_stream(sub.stream_id);
-    maybe_hide_topic_bracket(section_id);
+    if (section_id !== undefined) {
+        maybe_hide_topic_bracket(section_id);
+    }
 
     // Only scroll pinned topics into view.  If we're unpinning
     // a topic, we may be literally trying to get it out of
@@ -967,7 +969,7 @@ function on_sidebar_channel_click(
     }
 
     const section_for_stream = stream_list_sort.current_section_id_for_stream(stream_id);
-    if (collapsed_sections.has(section_for_stream)) {
+    if (section_for_stream !== undefined && collapsed_sections.has(section_for_stream)) {
         // In the event that user clicks on the channel in the left
         // sidebar when its folder is collapsed, which is only there
         // to click on if the user was already viewing that channel,
diff --git a/web/src/stream_list_sort.ts b/web/src/stream_list_sort.ts
index e43b711c7a..0d3458cfb8 100644
--- a/web/src/stream_list_sort.ts
+++ b/web/src/stream_list_sort.ts
@@ -49,11 +49,11 @@ function current_section_ids_for_streams(): Map<number, StreamListSection> {
     return map;
 }
 
-export function current_section_id_for_stream(stream_id: number): string {
+// Will return undefined if the given stream isn't in the user's stream
+// list (i.e. they're not subscribed to it).
+export function current_section_id_for_stream(stream_id: number): string | undefined {
     // Warning: This function is O(n), so it should not be called in a loop.
-    const section = current_section_ids_for_streams().get(stream_id);
-    assert(section !== undefined);
-    return section.id;
+    return current_section_ids_for_streams().get(stream_id)?.id;
 }
 
 function compare_function(a: number, b: number): number {
diff --git a/web/src/stream_settings_ui.ts b/web/src/stream_settings_ui.ts
index c6efab5145..097d0efefb 100644
--- a/web/src/stream_settings_ui.ts
+++ b/web/src/stream_settings_ui.ts
@@ -261,7 +261,9 @@ export function update_channel_folder(sub: StreamSubscription, folder_id: number
     stream_ui_updates.update_channel_folder_dropdown(sub);
     stream_list.build_stream_list(false);
     const section_id = stream_list_sort.current_section_id_for_stream(sub.stream_id);
-    stream_list.maybe_hide_topic_bracket(section_id);
+    if (section_id !== undefined) {
+        stream_list.maybe_hide_topic_bracket(section_id);
+    }
 }
 
 export function reset_dropdown_set_to_archived_folder(folder_id: number): void {