diff --git a/content/base/test/chrome/cpows_child.js b/content/base/test/chrome/cpows_child.js
index edfbf5f311d4..a53c7c1940d5 100644
--- a/content/base/test/chrome/cpows_child.js
+++ b/content/base/test/chrome/cpows_child.js
@@ -27,8 +27,15 @@ function make_object()
   o.b = true;
   o.s = "hello";
   o.x = { i: 10 };
-  o.f = function () { return 99; }
+  o.f = function () { return 99; };
+
+  // Doing anything with this Proxy will throw.
+  var throwing = new Proxy({}, new Proxy({}, {
+      get: function (trap) { throw trap; }
+    }));
+
   return { "data": o,
+           "throwing": throwing,
            "document": content.document
          };
 }
diff --git a/content/base/test/chrome/cpows_parent.xul b/content/base/test/chrome/cpows_parent.xul
index 52461b42bcf2..7d9592a8f96f 100644
--- a/content/base/test/chrome/cpows_parent.xul
+++ b/content/base/test/chrome/cpows_parent.xul
@@ -37,6 +37,39 @@
       ok(data.b === false, "boolean property");
       ok(data.s === "bye", "string property");
       ok(data.x === null, "nested property");
+
+      let throwing = message.objects.throwing;
+      // Based on the table on:
+      // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy
+      let tests = [
+        () => Object.getOwnPropertyDescriptor(throwing, 'test'),
+        () => Object.getOwnPropertyNames(throwing),
+        () => Object.defineProperty(throwing, 'test', {value: 1}),
+        () => delete throwing.test,
+        () => "test" in throwing,
+        () => Object.prototype.hasOwnProperty.call(throwing, 'test'),
+        () => throwing.test,
+        () => { throwing.test = 1 },
+        // () => { for (let prop in throwing) {} }, Bug 783829
+        () => { for (let prop of throwing) {} },
+        () => Object.keys(throwing),
+        () => Function.prototype.call.call(throwing),
+        () => new throwing,
+        () => Object.preventExtensions(throwing),
+        () => Object.freeze(throwing),
+        () => Object.seal(throwing),
+      ]
+
+      for (let test of tests) {
+        let threw = false;
+        try {
+          test()
+        } catch (e) {
+          threw = true;
+        }
+        ok(threw, "proxy operation threw exception");
+      }
+
     }
 
     function recvAsyncMessage(message) {
@@ -57,7 +90,7 @@
         run_tests("inprocess");
         return;
       }
-        
+
       finish();
     }
 
diff --git a/js/ipc/JavaScriptChild.cpp b/js/ipc/JavaScriptChild.cpp
index 9ee02c5eebaa..52bcfc904726 100644
--- a/js/ipc/JavaScriptChild.cpp
+++ b/js/ipc/JavaScriptChild.cpp
@@ -434,7 +434,7 @@ JavaScriptChild::AnswerIsExtensible(const ObjectId &objId, ReturnStatus *rs, boo
         return fail(cx, rs);
 
     *result = !!extensible;
-    return true;
+    return ok(rs);
 }
 
 bool
diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index 57711331d614..76dc052a843e 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -7206,12 +7206,11 @@ JS_GetScriptedGlobal(JSContext *cx)
 JS_PUBLIC_API(JSBool)
 JS_PreventExtensions(JSContext *cx, JS::HandleObject obj)
 {
-    JSBool extensible;
-    if (!JS_IsExtensible(cx, obj, &extensible))
-        return JS_TRUE;
-    if (extensible)
-        return JS_TRUE;
-
+    bool extensible;
+    if (!JSObject::isExtensible(cx, obj, &extensible))
+        return false;
+    if (!extensible)
+        return true;
     return JSObject::preventExtensions(cx, obj);
 }