diff --git a/tests/integration/btoa/btoa.js b/tests/integration/btoa/btoa.js
index 79ecf81..f9a543a 100644
--- a/tests/integration/btoa/btoa.js
+++ b/tests/integration/btoa/btoa.js
@@ -374,5 +374,6 @@ export const handler = serveTest(async () => {
     throws(() => atob("~~"));
     throws(() => atob(".."));
     throws(() => atob("--"));
+    throws(() => atob("__"));
   }
 });