diff --git a/src/shared/shared.ts b/src/shared/shared.ts
index 1c3ba524..44f562a9 100644
--- a/src/shared/shared.ts
+++ b/src/shared/shared.ts
@@ -191,7 +191,7 @@ const KNOWN_EXTENSIONS = new Set(
 )
 
 export function treatAsHtml(filename: string): boolean {
-  const ext = filename.split('.').pop()?.toLowerCase()
+  const ext = filename.split('.').pop()
 
-  return ext == null || !KNOWN_EXTENSIONS.has(ext)
+  return ext == null || !KNOWN_EXTENSIONS.has(ext.toLowerCase())
 }