diff --git a/test/built-ins/Uint8Array/prototype/setFromBase64/throws-when-target-is-backed-by-immutable-arraybuffer.js b/test/built-ins/Uint8Array/prototype/setFromBase64/throws-when-target-is-backed-by-immutable-arraybuffer.js
new file mode 100644
index 00000000000..2984c2b4e38
--- /dev/null
+++ b/test/built-ins/Uint8Array/prototype/setFromBase64/throws-when-target-is-backed-by-immutable-arraybuffer.js
@@ -0,0 +1,19 @@
+// Copyright (C) 2026 Kevin Gibbons. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-uint8array.prototype.setfrombase64
+description: Uint8Array.prototype.setFromBase64 throws a TypeError when the target is backed by an immutable ArrayBuffer.
+features: [ArrayBuffer, TypedArray, uint8array-base64, immutable-arraybuffer]
+---*/
+
+var iab = (new ArrayBuffer(10)).transferToImmutable();
+var target = new Uint8Array(iab);
+
+assert.throws(TypeError, function () {
+  target.setFromBase64('Zg==');
+});
+
+assert.throws(TypeError, function () {
+  target.setFromBase64('');
+});
diff --git a/test/built-ins/Uint8Array/prototype/setFromHex/throws-when-target-is-backed-by-immutable-arraybuffer.js b/test/built-ins/Uint8Array/prototype/setFromHex/throws-when-target-is-backed-by-immutable-arraybuffer.js
new file mode 100644
index 00000000000..3b50a13559d
--- /dev/null
+++ b/test/built-ins/Uint8Array/prototype/setFromHex/throws-when-target-is-backed-by-immutable-arraybuffer.js
@@ -0,0 +1,19 @@
+// Copyright (C) 2026 Kevin Gibbons. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-uint8array.prototype.setfromhex
+description: Uint8Array.prototype.setFromHex throws a TypeError when the target is backed by an immutable ArrayBuffer.
+features: [ArrayBuffer, TypedArray, uint8array-base64, immutable-arraybuffer]
+---*/
+
+var iab = (new ArrayBuffer(10)).transferToImmutable();
+var target = new Uint8Array(iab);
+
+assert.throws(TypeError, function () {
+  target.setFromHex('aa');
+});
+
+assert.throws(TypeError, function () {
+  target.setFromHex('');
+});