diff --git a/test/built-ins/Proxy/isExtensible/trap-is-missing-target-is-proxy.js b/test/built-ins/Proxy/isExtensible/trap-is-missing-target-is-proxy.js
new file mode 100644
index 0000000000..7d2516ab77
--- /dev/null
+++ b/test/built-ins/Proxy/isExtensible/trap-is-missing-target-is-proxy.js
@@ -0,0 +1,31 @@
+// Copyright (C) 2020 Alexey Shvayka. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-proxy-object-internal-methods-and-internal-slots-isextensible
+description: >
+  If "isExtensible" trap is null or undefined, [[IsExtensible]] call is
+  properly forwarded to [[ProxyTarget]] (which is also a Proxy object).
+info: |
+  [[IsExtensible]] ( )
+
+  [...]
+  4. Let target be O.[[ProxyTarget]].
+  5. Let trap be ? GetMethod(handler, "isExtensible").
+  6. If trap is undefined, then
+    a. Return ? IsExtensible(target).
+
+  IsExtensible ( O )
+
+  1. Assert: Type(O) is Object.
+  2. Return ? O.[[IsExtensible]]().
+features: [Proxy]
+---*/
+
+var regExp = /(?:)/g;
+Object.preventExtensions(regExp);
+
+var regExpTarget = new Proxy(regExp, {});
+var regExpProxy = new Proxy(regExpTarget, {});
+
+assert(!Object.isExtensible(regExpProxy));
diff --git a/test/built-ins/Proxy/isExtensible/trap-is-null-target-is-proxy.js b/test/built-ins/Proxy/isExtensible/trap-is-null-target-is-proxy.js
new file mode 100644
index 0000000000..3c2616da9f
--- /dev/null
+++ b/test/built-ins/Proxy/isExtensible/trap-is-null-target-is-proxy.js
@@ -0,0 +1,30 @@
+// Copyright (C) 2020 Alexey Shvayka. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-proxy-object-internal-methods-and-internal-slots-isextensible
+description: >
+  If "isExtensible" trap is null or undefined, [[IsExtensible]] call is
+  properly forwarded to [[ProxyTarget]] (which is also a Proxy object).
+info: |
+  [[IsExtensible]] ( )
+
+  [...]
+  4. Let target be O.[[ProxyTarget]].
+  5. Let trap be ? GetMethod(handler, "isExtensible").
+  6. If trap is undefined, then
+    a. Return ? IsExtensible(target).
+
+  IsExtensible ( O )
+
+  1. Assert: Type(O) is Object.
+  2. Return ? O.[[IsExtensible]]().
+features: [Proxy]
+---*/
+
+var plainObjectTarget = new Proxy({}, {});
+var plainObjectProxy = new Proxy(plainObjectTarget, {
+  isExtensible: null,
+});
+
+assert(Object.isExtensible(plainObjectProxy));
diff --git a/test/built-ins/Proxy/isExtensible/trap-is-undefined-target-is-proxy.js b/test/built-ins/Proxy/isExtensible/trap-is-undefined-target-is-proxy.js
new file mode 100644
index 0000000000..3f390af65b
--- /dev/null
+++ b/test/built-ins/Proxy/isExtensible/trap-is-undefined-target-is-proxy.js
@@ -0,0 +1,49 @@
+// Copyright (C) 2020 Alexey Shvayka. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-proxy-object-internal-methods-and-internal-slots-isextensible
+description: >
+  If "isExtensible" trap is null or undefined, [[IsExtensible]] call is
+  properly forwarded to [[ProxyTarget]] (which is also a Proxy object).
+info: |
+  [[IsExtensible]] ( )
+
+  [...]
+  4. Let target be O.[[ProxyTarget]].
+  5. Let trap be ? GetMethod(handler, "isExtensible").
+  6. If trap is undefined, then
+    a. Return ? IsExtensible(target).
+
+  IsExtensible ( O )
+
+  1. Assert: Type(O) is Object.
+  2. Return ? O.[[IsExtensible]]().
+
+  [[IsExtensible]] ( )
+
+  [...]
+  7. Let booleanTrapResult be ! ToBoolean(? Call(trap, handler, « target »)).
+  [...]
+  10. Return booleanTrapResult.
+features: [Proxy]
+---*/
+
+var array = [];
+var arrayExtensible = true;
+var arrayTarget = new Proxy(array, {
+  isExtensible: function() {
+    return arrayExtensible;
+  },
+});
+
+var arrayProxy = new Proxy(arrayTarget, {
+  isExtensible: undefined,
+});
+
+assert(Object.isExtensible(arrayProxy));
+
+Object.preventExtensions(array);
+arrayExtensible = false;
+
+assert(!Object.isExtensible(arrayProxy));