diff --git a/dialog/internal/dialog.ts b/dialog/internal/dialog.ts
index cc86fd33c0..a8b610beae 100644
--- a/dialog/internal/dialog.ts
+++ b/dialog/internal/dialog.ts
@@ -153,7 +153,7 @@ export class Dialog extends LitElement {
   // focusable elements. TreeWalker is faster than `querySelectorAll('*')`.
   // We check for isServer because there isn't a "document" during an SSR
   // run.
-  private readonly treewalker = isServer ? {} : document.createTreeWalker(
+  private readonly treewalker = isServer ? {} as TreeWalker : document.createTreeWalker(
     this,
     NodeFilter.SHOW_ELEMENT,
   );