diff --git a/web_src/js/features/repo-issue-pull.ts b/web_src/js/features/repo-issue-pull.ts
index 77948c4edd..34cc4ed199 100644
--- a/web_src/js/features/repo-issue-pull.ts
+++ b/web_src/js/features/repo-issue-pull.ts
@@ -52,7 +52,12 @@ function initRepoPullMergeBoxRefresh(el: Element) {
       const pullLink = el.getAttribute('data-pull-link')!;
       const resp = await GET(`${pullLink}/merge_box`);
       if (!resp.ok) return;
-      const newEl = createElementFromHTML(await resp.text());
+      const respText = (await resp.text()).trim();
+      if (!respText) {
+        el.remove(); // merge box might not exist if the PR has changed (e.g.: merged and the head branch has been deleted)
+        return;
+      }
+      const newEl = createElementFromHTML(respText);
       const scrollTop = el.querySelector<HTMLElement>('.commit-status-list')?.scrollTop;
       el.replaceWith(newEl); // don't morph, do full replacement to make sure data-global-init and Vue components are re-initialized
       if (scrollTop) newEl.querySelector<HTMLElement>('.commit-status-list')?.scrollTo({top: scrollTop, behavior: 'instant'});
diff --git a/web_src/js/utils/dom.ts b/web_src/js/utils/dom.ts
index 3fa865645d..71b6b7506f 100644
--- a/web_src/js/utils/dom.ts
+++ b/web_src/js/utils/dom.ts
@@ -267,10 +267,10 @@ export function isElemVisible(el: HTMLElement): boolean {
 
 export function createElementFromHTML<T extends Element>(htmlString: string): T {
   htmlString = htmlString.trim();
+  if (!htmlString.startsWith('<')) throw new Error(`Invalid HTML element string: ${htmlString}`);
   const isLetter = (code: number) => (code >= 65 && code <= 90) || (code >= 97 && code <= 122);
   const startsWithTag = (s: string, tag: string) => {
-    return s.startsWith('<') &&
-      s.substring(1, 1 + tag.length).toLowerCase() === tag.toLowerCase() &&
+    return s.substring(1, 1 + tag.length).toLowerCase() === tag.toLowerCase() &&
       !isLetter(s[1 + tag.length].charCodeAt(0));
   };
   // There is no way to create some elements without a proper parent, jQuery's approach: https://github.com/jquery/jquery/blob/main/src/manipulation/wrapMap.js