diff --git a/features.txt b/features.txt
index 79f909f462..595e470733 100644
--- a/features.txt
+++ b/features.txt
@@ -88,6 +88,10 @@ upsert
 # https://github.com/tc39/proposal-immutable-arraybuffer
 immutable-arraybuffer
 
+# Non-extensible Applies to Private
+# https://github.com/tc39/proposal-nonextensible-applies-to-private
+nonextensible-applies-to-private
+
 ## Standard language features
 #
 # Language features that have been included in a published version of the
diff --git a/test/language/statements/class/elements/private-class-field-on-frozen-objects.js b/test/language/statements/class/elements/private-class-field-on-frozen-objects.js
deleted file mode 100644
index 9517976c10..0000000000
--- a/test/language/statements/class/elements/private-class-field-on-frozen-objects.js
+++ /dev/null
@@ -1,31 +0,0 @@
-// Copyright (C) 2019 Caio Lima. All rights reserved.
-// This code is governed by the BSD license found in the LICENSE file.
-
-/*---
-description: It is possible to add private fields on frozen objects
-esid: sec-define-field
-info: |
-  DefineField(receiver, fieldRecord)
-    ...
-    8. If fieldName is a Private Name,
-      a. Perform ? PrivateFieldAdd(fieldName, receiver, initValue).
-    9. Else,
-      a. Assert: IsPropertyKey(fieldName) is true.
-      b. Perform ? CreateDataPropertyOrThrow(receiver, fieldName, initValue).
-    10. Return.
-features: [class, class-fields-private, class-fields-public]
-flags: [onlyStrict]
----*/
-
-class Test {
-  f = this;
-  #g = (Object.freeze(this), "Test262");
-
-  get g() {
-    return this.#g;
-  }
-}
-
-let t = new Test();
-assert.sameValue(t.f, t);
-assert.sameValue(t.g, "Test262");
diff --git a/test/language/statements/class/elements/private-class-field-on-nonextensible-objects.js b/test/language/statements/class/elements/private-class-field-on-nonextensible-objects.js
new file mode 100644
index 0000000000..ada3868916
--- /dev/null
+++ b/test/language/statements/class/elements/private-class-field-on-nonextensible-objects.js
@@ -0,0 +1,117 @@
+// Copyright (C) 2019 Caio Lima. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: It is not possible to add private fields on non-extensible objects
+esid: sec-define-field
+info: |
+  1.1 PrivateFieldAdd ( O, P, value )
+    1. If O.[[Extensible]] is false, throw a TypeError exception.
+    ...
+
+features:
+  - class
+  - class-fields-private
+  - class-fields-public
+  - nonextensible-applies-to-private
+flags: [onlyStrict]
+---*/
+
+// Analogous to
+// test/language/statements/class/subclass/private-class-field-on-nonextensible-return-override.js
+
+class NonExtensibleBase {
+  constructor(seal) {
+    if (seal) Object.preventExtensions(this);
+  }
+}
+
+
+// extend superclass with private instance data field
+class ClassWithPrivateField extends NonExtensibleBase {
+  #val;
+
+  constructor(seal) {
+    super(seal);
+    this.#val = 42;
+  }
+  val() {
+    return this.#val;
+  }
+}
+
+const t = new ClassWithPrivateField(false);
+// extensible objects can be extended
+assert.sameValue(t.val(), 42);
+
+// where superclass prevented extensions & subclass extended
+assert.throws(TypeError, function () {
+  new ClassWithPrivateField(true);
+});
+
+
+// extend superclass with private instance method
+class ClassWithPrivateMethod extends NonExtensibleBase {
+  constructor(seal) {
+    super(seal);
+  }
+  // private methods are on the instance, so will fail
+  #privateMethod() {
+    return 42;
+  };
+  // public methods are on the prototype, so are ok.
+  publicMethod() {
+    return this.#privateMethod();
+  }
+}
+
+const m = new ClassWithPrivateMethod(false);
+// extensible objects can be extended
+assert.sameValue(m.publicMethod(), 42);
+
+// where superclass prevented extensions & subclass extended
+assert.throws(TypeError, function () {
+  new ClassWithPrivateMethod(true);
+});
+
+
+// extend superclass with private instance accessor
+class ClassWithPrivateAccessor extends NonExtensibleBase {
+  constructor(seal) {
+    super(seal);
+  }
+  // private accessors are on the instance, so will fail
+  get #privateAccessor() {
+    return 42;
+  };
+  // public accessors are on the prototype, so are ok.
+  get publicAccessor() {
+    return this.#privateAccessor;
+  }
+}
+
+const a = new ClassWithPrivateAccessor(false);
+// extensible objects can be extended
+assert.sameValue(m.publicAccessor, 42);
+
+// where superclass prevented extensions & subclass extended
+assert.throws(TypeError, function () {
+  new ClassWithPrivateAccessor(true);
+});
+
+
+// base class private instance data field
+class TestNonExtensibleData {
+  #g = (Object.preventExtensions(this), "Test262");
+}
+
+assert.throws(TypeError, function () {
+  new TestNonExtensibleData();
+});
+
+// base class with private static data field
+assert.throws(TypeError, function () {
+  class TestNonExtensibleStaticData {
+    static #g = (Object.preventExtensions(TestNonExtensibleStaticData), "Test262");
+  }
+});
diff --git a/test/language/statements/class/subclass/private-class-field-on-nonextensible-return-override.js b/test/language/statements/class/subclass/private-class-field-on-nonextensible-return-override.js
new file mode 100644
index 0000000000..d8e7034a7d
--- /dev/null
+++ b/test/language/statements/class/subclass/private-class-field-on-nonextensible-return-override.js
@@ -0,0 +1,100 @@
+// Copyright (C) 2019 Caio Lima. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: It is not possible to add private fields on non-extensible objects via return override
+esid: sec-define-field
+info: |
+  1.1 PrivateFieldAdd ( O, P, value )
+    1. If O.[[Extensible]] is false, throw a TypeError exception.
+    ...
+
+features:
+  - class
+  - class-fields-private
+  - class-fields-public
+  - nonextensible-applies-to-private
+flags: [onlyStrict]
+---*/
+
+// Analogous to
+// test/language/statements/class/elements/private-class-field-on-nonextensible-objects.js
+
+class TrojanBase {
+  constructor(obj) {
+    return obj;
+  }
+}
+
+
+// extend superclass with private instance data field
+class ClassWithPrivateField extends TrojanBase {
+  #val;
+
+  constructor(obj) {
+    super(obj);
+    this.#val = 42;
+  }
+  val() {
+    return this.#val;
+  }
+}
+
+const t = new ClassWithPrivateField({});
+// extensible objects can be extended
+assert.sameValue(t.val(), 42);
+
+// where superclass prevented extensions & subclass extended
+assert.throws(TypeError, function () {
+  new ClassWithPrivateField(Object.preventExtensions({}));
+});
+
+
+// extend superclass with private instance method
+class ClassWithPrivateMethod extends TrojanBase {
+  constructor(obj) {
+    super(obj);
+  }
+  // private methods are on the instance, so will fail
+  #privateMethod() {
+    return 42;
+  };
+  // public methods are on the prototype, so are ok.
+  publicMethod() {
+    return this.#privateMethod();
+  }
+}
+
+const m = new ClassWithPrivateMethod({});
+// extensible objects can be extended
+assert.sameValue(m.publicMethod(), 42);
+
+// where superclass prevented extensions & subclass extended
+assert.throws(TypeError, function () {
+  new ClassWithPrivateMethod(Object.preventExtensions({}));
+});
+
+
+// extend superclass with private instance accessor
+class ClassWithPrivateAccessor extends TrojanBase {
+  constructor(obj) {
+    super(obj);
+  }
+  // private accessors are on the instance, so will fail
+  get #privateAccessor() {
+    return 42;
+  };
+  // public accessors are on the prototype, so are ok.
+  get publicAccessor() {
+    return this.#privateAccessor;
+  }
+}
+
+const a = new ClassWithPrivateAccessor({});
+// extensible objects can be extended
+assert.sameValue(m.publicAccessor, 42);
+
+// where superclass prevented extensions & subclass extended
+assert.throws(TypeError, function () {
+  new ClassWithPrivateAccessor(Object.preventExtensions({}));
+});
diff --git a/test/staging/sm/PrivateName/modify-non-extensible.js b/test/staging/sm/PrivateName/modify-non-extensible.js
index b3e7283cbb..f122f9a3b8 100644
--- a/test/staging/sm/PrivateName/modify-non-extensible.js
+++ b/test/staging/sm/PrivateName/modify-non-extensible.js
@@ -2,6 +2,11 @@
 // This code is governed by the BSD license found in the LICENSE file.
 
 /*---
+features:
+  - class
+  - class-fields-private
+  - class-fields-public
+  - nonextensible-applies-to-private
 flags:
   - noStrict
 description: |
@@ -32,8 +37,8 @@ class A extends OverrideBase {
 }
 
 var obj = {};
-Object.seal(obj);
 new A(obj);  // Add #a to obj, but not g.
+Object.seal(obj);
 assert.sameValue('g' in obj, false);
 assert.sameValue(A.gs(obj), 1);
 A.inca(obj);
@@ -67,8 +72,10 @@ assert.sameValue(A.gs(proxy), 2)
 
 var target = { a: 10 };
 Object.freeze(target);
-new A(target);
-assert.sameValue(Object.isFrozen(target), true)
+assert.throws(TypeError, function () {
+  new A(target);
+});
+assert.sameValue(Object.isFrozen(target), true);
 
 var getOwnKeys = [];
 var proxy = new Proxy(target, {
@@ -80,4 +87,3 @@ var proxy = new Proxy(target, {
 
 Object.isFrozen(proxy);
 assert.sameValue(getOwnKeys.length, 1);
-