diff --git a/dom/browser-element/mochitest/browserElement_ExecuteScript.js b/dom/browser-element/mochitest/browserElement_ExecuteScript.js
index 3b217816b5df..334a4f8791cd 100644
--- a/dom/browser-element/mochitest/browserElement_ExecuteScript.js
+++ b/dom/browser-element/mochitest/browserElement_ExecuteScript.js
@@ -91,7 +91,7 @@ function runTest() {
       ok(c(rv, 'YQ=='), `scriptId: ${scriptId++}`);
       return iframe.executeScript('window.wrappedJSObject.btoa("a")', {url})
     }, bail).then(bail, (error) => {
-      is(error.message, `TypeError: window.wrappedJSObject is undefined, can't access its "btoa" property`, `scriptId: ${scriptId++}`);
+      is(error.message, `TypeError: window.wrappedJSObject is undefined, can't access property "btoa" of it`, `scriptId: ${scriptId++}`);
       return iframe.executeScript('42', {})
     }).then(bail, error => {
       is(error.name, 'InvalidAccessError', `scriptId: ${scriptId++}`);
diff --git a/js/src/jit-test/tests/basic/bug827104.js b/js/src/jit-test/tests/basic/bug827104.js
index a506f5ff7412..343fcebcd596 100644
--- a/js/src/jit-test/tests/basic/bug827104.js
+++ b/js/src/jit-test/tests/basic/bug827104.js
@@ -10,4 +10,4 @@ var e;
 try {
     f();
 } catch (error) {e = error;}
-assertEq(e.toString(), `TypeError: a[i] is undefined; can't access element at index 0`);
+assertEq(e.toString(), `TypeError: a[i] is undefined, can't access property 0 of it`);
diff --git a/js/src/jit-test/tests/basic/expression-autopsy.js b/js/src/jit-test/tests/basic/expression-autopsy.js
index 06a7e0c3bc33..c3a145827ff1 100644
--- a/js/src/jit-test/tests/basic/expression-autopsy.js
+++ b/js/src/jit-test/tests/basic/expression-autopsy.js
@@ -18,7 +18,7 @@ function check_one(expected, f, err) {
 ieval = eval;
 function check(expr, expected=expr, testStrict=true) {
     var end, err;
-    for ([end, err] of [[".random_prop", ` is undefined; can't access its "random_prop" property`], ["()", " is not a function"]]) {
+    for ([end, err] of [[".random_prop", ` is undefined, can't access property \"random_prop" of it`], ["()", " is not a function"]]) {
         var statement = "o = {};" + expr + end, f;
         var cases = [
             // Global scope
@@ -102,7 +102,7 @@ check_one("6", (function () { 6() }), " is not a function");
 check_one("4", (function() { (4||eval)(); }), " is not a function");
 check_one("0", (function () { Array.prototype.reverse.call('123'); }), " is read-only");
 check_one("[...][Symbol.iterator](...).next(...).value",
-          function () { ieval("{ let x; var [a, b, [c0, c1]] = [x, x, x]; }") }, " is undefined; can't access its Symbol.iterator property");
+          function () { ieval("{ let x; var [a, b, [c0, c1]] = [x, x, x]; }") }, " is undefined, can't access property Symbol.iterator of it");
 check_one("(void 1)", function() { (void 1)(); }, " is not a function");
 check_one("(void o[1])", function() { var o = []; (void o[1])() }, " is not a function");
 
diff --git a/js/src/jit-test/tests/basic/iterable-error-messages.js b/js/src/jit-test/tests/basic/iterable-error-messages.js
index b32b27e9c9b2..d783d15900c3 100644
--- a/js/src/jit-test/tests/basic/iterable-error-messages.js
+++ b/js/src/jit-test/tests/basic/iterable-error-messages.js
@@ -15,7 +15,7 @@ function testForOf(val) {
 for (v of [{}, Math, new Proxy({}, {})]) {
     assertThrowsMsg(() => testForOf(v), "val is not iterable");
 }
-assertThrowsMsg(() => testForOf(null), "val is null; can't access its Symbol.iterator property");
+assertThrowsMsg(() => testForOf(null), "val is null, can't access property Symbol.iterator of it");
 assertThrowsMsg(() => { for (var x of () => 1) {}}, "() => 1 is not iterable");
 
 // Destructuring
@@ -25,7 +25,7 @@ function testDestr(val) {
 for (v of [{}, Math, new Proxy({}, {})]) {
     assertThrowsMsg(() => testDestr(v), "val is not iterable");
 }
-assertThrowsMsg(() => testDestr(null), "val is null; can't access its Symbol.iterator property");
+assertThrowsMsg(() => testDestr(null), "val is null, can't access property Symbol.iterator of it");
 assertThrowsMsg(() => { [a, b] = () => 1; }, "() => 1 is not iterable");
 
 // Spread
@@ -35,5 +35,5 @@ function testSpread(val) {
 for (v of [{}, Math, new Proxy({}, {})]) {
     assertThrowsMsg(() => testSpread(v), "val is not iterable");
 }
-assertThrowsMsg(() => testSpread(null), "val is null; can't access its Symbol.iterator property");
+assertThrowsMsg(() => testSpread(null), "val is null, can't access property Symbol.iterator of it");
 assertThrowsMsg(() => { [...() => 1]; }, "() => 1 is not iterable");
diff --git a/js/src/jit-test/tests/basic/testBug604210.js b/js/src/jit-test/tests/basic/testBug604210.js
index f4119f016947..bb96e7c4963f 100644
--- a/js/src/jit-test/tests/basic/testBug604210.js
+++ b/js/src/jit-test/tests/basic/testBug604210.js
@@ -6,6 +6,6 @@ function f() {
     } catch (e) {
         msg = '' + e;
     }
-    assertEq(msg, `TypeError: x is undefined; can't access its "foo" property`);
+    assertEq(msg, `TypeError: x is undefined, can't access property "foo" of it`);
 }
 f();
diff --git a/js/src/jit-test/tests/debug/bug1275001.js b/js/src/jit-test/tests/debug/bug1275001.js
index 84d5a7013db1..868a7bda2cf5 100644
--- a/js/src/jit-test/tests/debug/bug1275001.js
+++ b/js/src/jit-test/tests/debug/bug1275001.js
@@ -17,13 +17,13 @@ function check_one(expected, f, err) {
 ieval = eval
 function check(expr, expected = expr) {
     var end, err
-    for ([end, err] of [[".random_prop", ` is undefined, can't access its "random_prop" property`]])
+    for ([end, err] of[[".random_prop", ` is undefined, can't access property \"random_prop" of it` ]]) 
          statement = "o = {};" + expr + end;
          cases = [
             function() { return ieval("var undef;" + statement); },
             Function(statement)
         ]
-        for (f of cases)
+        for (f of cases) 
             check_one(expected, f, err)
 }
 check("undef");
diff --git a/js/src/jit-test/tests/ion/bug913749.js b/js/src/jit-test/tests/ion/bug913749.js
index 068086e362d4..bece1e07d37c 100644
--- a/js/src/jit-test/tests/ion/bug913749.js
+++ b/js/src/jit-test/tests/ion/bug913749.js
@@ -15,7 +15,7 @@ for (var i = 0; i < 3; i++) {
 	x.toString();
 	assertEq(0, 1);
     } catch (e) {
-	assertEq(e.message === `y is undefined; can't access its "length" property` ||
+	assertEq(e.message === `y is undefined, can't access property "length" of it` ||
 		 e.message === "undefined has no properties", true);
     }
 }
diff --git a/js/src/js.msg b/js/src/js.msg
index 2158cc6f7662..92e5d4d0d9f7 100644
--- a/js/src/js.msg
+++ b/js/src/js.msg
@@ -9,22 +9,21 @@
  *
  * The format for each JS error message is:
  *
- *     MSG_DEF(<SYMBOLIC_NAME>, <ARGUMENT_COUNT>, <EXCEPTION_NAME>,
- *             <FORMAT_STRING>)
+ * MSG_DEF(<SYMBOLIC_NAME>, <ARGUMENT_COUNT>, <EXCEPTION_NAME>,
+ *         <FORMAT_STRING>)
  *
- * where:
+ * where ;
+ * <SYMBOLIC_NAME> is a legal C identifer that will be used in the
+ * JS engine source.
  *
- *     <SYMBOLIC_NAME> is a legal C identifer that will be used in the
- *     JS engine source.
+ * <ARGUMENT_COUNT> is an integer literal specifying the total number of
+ * replaceable arguments in the following format string.
  *
- *     <ARGUMENT_COUNT> is an integer literal specifying the total number of
- *     replaceable arguments in the following format string.
+ * <EXCEPTION_NAME> is an enum JSExnType value, defined in jsapi.h.
  *
- *     <EXCEPTION_NAME> is an enum JSExnType value, defined in jsapi.h.
- *
- *     <FORMAT_STRING> is a string literal, optionally containing sequences
- *     {X} where X  is an integer representing the argument number that will
- *     be replaced with a string value when the error is reported.
+ * <FORMAT_STRING> is a string literal, optionally containing sequences
+ * {X} where X  is an integer representing the argument number that will
+ * be replaced with a string value when the error is reported.
  *
  * e.g.
  *
@@ -56,8 +55,7 @@ MSG_DEF(JSMSG_TOPRIMITIVE_NOT_CALLABLE, 2, JSEXN_TYPEERR, "can't convert {0} to
 MSG_DEF(JSMSG_TOPRIMITIVE_RETURNED_OBJECT, 2, JSEXN_TYPEERR, "can't convert {0} to {1}: its [Symbol.toPrimitive] method returned an object")
 MSG_DEF(JSMSG_NO_PROPERTIES,           1, JSEXN_TYPEERR, "{0} has no properties")
 MSG_DEF(JSMSG_PROPERTY_FAIL,           2, JSEXN_TYPEERR, "can't access property {0} of {1}")
-MSG_DEF(JSMSG_PROPERTY_FAIL_EXPR,      3, JSEXN_TYPEERR, "{0} is {1}; can't access its {2} property")
-MSG_DEF(JSMSG_ELEMENT_FAIL_EXPR,       3, JSEXN_TYPEERR, "{0} is {1}; can't access element at index {2}")
+MSG_DEF(JSMSG_PROPERTY_FAIL_EXPR,      3, JSEXN_TYPEERR, "{0} is {1}, can't access property {2} of it")
 MSG_DEF(JSMSG_BAD_REGEXP_FLAG,         1, JSEXN_SYNTAXERR, "invalid regular expression flag {0}")
 MSG_DEF(JSMSG_INVALID_DATA_VIEW_LENGTH, 0, JSEXN_RANGEERR, "invalid data view length")
 MSG_DEF(JSMSG_OFFSET_LARGER_THAN_FILESIZE, 0, JSEXN_RANGEERR, "offset is larger than filesize")
diff --git a/js/src/jsapi-tests/testErrorInterceptor.cpp b/js/src/jsapi-tests/testErrorInterceptor.cpp
index e347a226e636..ab88abdfa233 100644
--- a/js/src/jsapi-tests/testErrorInterceptor.cpp
+++ b/js/src/jsapi-tests/testErrorInterceptor.cpp
@@ -34,25 +34,25 @@ BEGIN_TEST(testErrorInterceptor)
 {
     // Run the following snippets.
     const char* SAMPLES[] = {
-        "throw new Error('I am an Error')",
-        "throw new TypeError('I am a TypeError')",
-        "throw new ReferenceError('I am a ReferenceError')",
-        "throw new SyntaxError('I am a SyntaxError')",
-        "throw 5",
-        "undefined[0]",
-        "foo[0]",
-        "b[",
+        "throw new Error('I am an Error')\0",
+        "throw new TypeError('I am a TypeError')\0",
+        "throw new ReferenceError('I am a ReferenceError')\0",
+        "throw new SyntaxError('I am a SyntaxError')\0",
+        "throw 5\0",
+        "undefined[0]\0",
+        "foo[0]\0",
+        "b[\0",
     };
     // With the simpleInterceptor, we should end up with the following error:
     const char* TO_STRING[] = {
-        "Error: I am an Error",
-        "TypeError: I am a TypeError",
-        "ReferenceError: I am a ReferenceError",
-        "SyntaxError: I am a SyntaxError",
-        "5",
-        "TypeError: can't access property 0 of undefined",
-        "ReferenceError: foo is not defined",
-        "SyntaxError: expected expression, got end of script",
+        "Error: I am an Error\0",
+        "TypeError: I am a TypeError\0",
+        "ReferenceError: I am a ReferenceError\0",
+        "SyntaxError: I am a SyntaxError\0",
+        "5\0",
+        "TypeError: can't access property 0 of undefined\0",
+        "ReferenceError: foo is not defined\0",
+        "SyntaxError: expected expression, got end of script\0",
     };
     MOZ_ASSERT(mozilla::ArrayLength(SAMPLES) == mozilla::ArrayLength(TO_STRING));
 
diff --git a/js/src/tests/non262/extensions/regress-353116.js b/js/src/tests/non262/extensions/regress-353116.js
index 2debc2604587..0ec70e977b78 100644
--- a/js/src/tests/non262/extensions/regress-353116.js
+++ b/js/src/tests/non262/extensions/regress-353116.js
@@ -45,7 +45,7 @@ function test()
   }
   reportCompare(expect, actual, summary);
 
-  expect = `TypeError: x is undefined; can't access its "y" property`;
+  expect = `TypeError: x is undefined, can't access property "y" of it`;
   actual = 'No Error';
 
   try
@@ -59,7 +59,7 @@ function test()
   }
   reportCompare(expect, actual, summary);
 
-  expect = `TypeError: x is null; can't access its "y" property`;
+  expect = `TypeError: x is null, can't access property "y" of it`;
   actual = 'No Error';
 
   try
diff --git a/js/src/tests/non262/regress/regress-469625-03.js b/js/src/tests/non262/regress/regress-469625-03.js
index f814ef0b8bc5..a210b0625b11 100644
--- a/js/src/tests/non262/regress/regress-469625-03.js
+++ b/js/src/tests/non262/regress/regress-469625-03.js
@@ -25,7 +25,7 @@ function test()
     var [a, b, [c0, c1]] = [x, x, x];
   }
 
-  expect = `TypeError: [...][Symbol.iterator](...).next(...).value is null; can't access its Symbol.iterator property`;
+  expect = `TypeError: [...][Symbol.iterator](...).next(...).value is null, can't access property Symbol.iterator of it`;
   actual = 'No Error';
   try
   {
diff --git a/js/src/tests/non262/regress/regress-469758.js b/js/src/tests/non262/regress/regress-469758.js
index 2cc892e74478..9fe08149d781 100644
--- a/js/src/tests/non262/regress/regress-469758.js
+++ b/js/src/tests/non262/regress/regress-469758.js
@@ -9,6 +9,6 @@ try {
     err = e;
 }
 assertEq(err instanceof TypeError, true);
-assertEq(err.message, "[][j] is undefined; can't access element at index 2");
+assertEq(err.message, "[][j] is undefined, can't access property 2 of it");
 
 reportCompare(0, 0, 'ok');
diff --git a/js/src/vm/JSContext.cpp b/js/src/vm/JSContext.cpp
index f32d43eb0c72..d3a37f1a4373 100644
--- a/js/src/vm/JSContext.cpp
+++ b/js/src/vm/JSContext.cpp
@@ -953,15 +953,13 @@ js::ReportIsNullOrUndefinedForPropertyAccess(JSContext* cx, HandleValue v, Handl
     if (strcmp(bytes.get(), js_undefined_str) == 0 || strcmp(bytes.get(), js_null_str) == 0) {
         JS_ReportErrorNumberUTF8(cx, GetErrorMessage, nullptr, JSMSG_PROPERTY_FAIL,
                                  keyBytes.get(), bytes.get());
+    } else if (v.isUndefined()) {
+        JS_ReportErrorNumberUTF8(cx, GetErrorMessage, nullptr, JSMSG_PROPERTY_FAIL_EXPR,
+                                 bytes.get(), js_undefined_str, keyBytes.get());
     } else {
-        const char* actual = v.isUndefined() ? js_undefined_str : js_null_str;
-        if (JSID_IS_INT(key)) {
-            JS_ReportErrorNumberUTF8(cx, GetErrorMessage, nullptr, JSMSG_ELEMENT_FAIL_EXPR,
-                                     bytes.get(), actual, keyBytes.get());
-        } else {
-            JS_ReportErrorNumberUTF8(cx, GetErrorMessage, nullptr, JSMSG_PROPERTY_FAIL_EXPR,
-                                     bytes.get(), actual, keyBytes.get());
-        }
+        MOZ_ASSERT(v.isNull());
+        JS_ReportErrorNumberUTF8(cx, GetErrorMessage, nullptr, JSMSG_PROPERTY_FAIL_EXPR,
+                                 bytes.get(), js_null_str, keyBytes.get());
     }
 }
 
diff --git a/toolkit/components/extensions/test/xpcshell/test_ext_contentscript_create_iframe.js b/toolkit/components/extensions/test/xpcshell/test_ext_contentscript_create_iframe.js
index c4593d105140..bd53c5e8896c 100644
--- a/toolkit/components/extensions/test/xpcshell/test_ext_contentscript_create_iframe.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_contentscript_create_iframe.js
@@ -125,7 +125,7 @@ add_task(async function test_contentscript_create_iframe() {
     Assert.ok(!manifest, "manifest should be undefined");
 
     Assert.equal(String(manifestException),
-                 `TypeError: win.browser.runtime is undefined, can't access its "getManifest" property`,
+                 `TypeError: win.browser.runtime is undefined, can't access property "getManifest" of it`,
                  "expected exception received");
 
     let getManifestException = win.testGetManifestException();