diff --git a/web/src/scroll_util.ts b/web/src/scroll_util.ts
index a4be25b85a..ded4918427 100644
--- a/web/src/scroll_util.ts
+++ b/web/src/scroll_util.ts
@@ -1,4 +1,5 @@
 import $ from "jquery";
+import assert from "minimalistic-assert";
 import SimpleBar from "simplebar";
 
 // This type is helpful for testing, where we may have a dummy object instead of an actual jquery object.
@@ -6,9 +7,10 @@ type JQueryOrZJQuery = {__zjquery?: true} & JQuery;
 
 export function get_content_element($element: JQuery): JQuery {
     const element = $element.expectOne()[0];
-    const sb = SimpleBar.instances.get(element);
+    const sb: unknown = SimpleBar.instances.get(element);
     if (sb) {
-        return $(sb.getContentElement());
+        assert(sb instanceof SimpleBar); // https://github.com/Grsmto/simplebar/pull/689
+        return $(sb.getContentElement()!);
     }
     return $element;
 }
@@ -20,9 +22,10 @@ export function get_scroll_element($element: JQueryOrZJQuery): JQuery {
     }
 
     const element = $element.expectOne()[0];
-    const sb = SimpleBar.instances.get(element);
+    const sb: unknown = SimpleBar.instances.get(element);
     if (sb) {
-        return $(sb.getScrollElement());
+        assert(sb instanceof SimpleBar); // https://github.com/Grsmto/simplebar/pull/689
+        return $(sb.getScrollElement()!);
     } else if ("simplebar" in element.dataset) {
         // The SimpleBar mutation observer hasn’t processed this element yet.
         // Create the SimpleBar early in case we need to add event listeners.
@@ -33,9 +36,10 @@ export function get_scroll_element($element: JQueryOrZJQuery): JQuery {
 
 export function reset_scrollbar($element: JQuery): void {
     const element = $element.expectOne()[0];
-    const sb = SimpleBar.instances.get(element);
+    const sb: unknown = SimpleBar.instances.get(element);
     if (sb) {
-        sb.getScrollElement().scrollTop = 0;
+        assert(sb instanceof SimpleBar); // https://github.com/Grsmto/simplebar/pull/689
+        sb.getScrollElement()!.scrollTop = 0;
     } else {
         element.scrollTop = 0;
     }