diff --git a/src/org/mozilla/javascript/ScriptRuntime.java b/src/org/mozilla/javascript/ScriptRuntime.java
index f6641cf23a..2a02469156 100644
--- a/src/org/mozilla/javascript/ScriptRuntime.java
+++ b/src/org/mozilla/javascript/ScriptRuntime.java
@@ -1476,6 +1476,10 @@ static Function getExistingCtor(Context cx, Scriptable scope, String constructor
      * Return -1L if str is not an index, or the index value as lower 32 bits of the result. Note
      * that the result needs to be cast to an int in order to produce the actual index, which may be
      * negative.
+     *
+     * <p>Note that this method on its own does not actually produce an index that is useful for an
+     * actual Object or Array, because it may be larger than Integer.MAX_VALUE. Most callers should
+     * instead call toStringOrIndex, which calls this under the covers.
      */
     public static long indexFromString(String str) {
         // The length of the decimal string representation of
diff --git a/src/org/mozilla/javascript/json/JsonParser.java b/src/org/mozilla/javascript/json/JsonParser.java
index b07f2ee0f7..e00b0ac7ed 100644
--- a/src/org/mozilla/javascript/json/JsonParser.java
+++ b/src/org/mozilla/javascript/json/JsonParser.java
@@ -10,6 +10,7 @@
 import java.util.List;
 import org.mozilla.javascript.Context;
 import org.mozilla.javascript.ScriptRuntime;
+import org.mozilla.javascript.ScriptRuntime.StringIdOrIndex;
 import org.mozilla.javascript.Scriptable;
 
 /**
@@ -118,13 +119,12 @@ private Object readObject() throws ParseException {
                     consume(':');
                     value = readValue();
 
-                    long index = ScriptRuntime.indexFromString(id);
-                    if (index < 0) {
-                        object.put(id, object, value);
+                    StringIdOrIndex indexObj = ScriptRuntime.toStringIdOrIndex(id);
+                    if (indexObj.getStringId() == null) {
+                        object.put(indexObj.getIndex(), object, value);
                     } else {
-                        object.put((int) index, object, value);
+                        object.put(indexObj.getStringId(), object, value);
                     }
-
                     needsComma = true;
                     break;
                 default:
diff --git a/testsrc/org/mozilla/javascript/tests/json/JsonParserTest.java b/testsrc/org/mozilla/javascript/tests/json/JsonParserTest.java
index 5ce6a09000..65498905dc 100644
--- a/testsrc/org/mozilla/javascript/tests/json/JsonParserTest.java
+++ b/testsrc/org/mozilla/javascript/tests/json/JsonParserTest.java
@@ -4,6 +4,7 @@
 
 package org.mozilla.javascript.tests.json;
 
+import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertEquals;
 
 import org.junit.After;
@@ -119,7 +120,6 @@ public void shouldParseEmptyJsonArray() throws Exception {
     }
 
     @Test
-    @SuppressWarnings("unchecked")
     public void shouldParseHeterogeneousJsonArray() throws Exception {
         NativeArray actual = (NativeArray) parser.parseValue("[ \"hello\" , 3, null, [false] ]");
         assertEquals("hello", actual.get(0, actual));
@@ -138,18 +138,37 @@ public void shouldFailToParseArrayWithInvalidElements() throws Exception {
     }
 
     @Test
-    @SuppressWarnings({"serial", "unchecked"})
     public void shouldParseJsonObject() throws Exception {
         String json =
                 "{" + "\"bool\" : false, " + "\"str\"  : \"xyz\", " + "\"obj\"  : {\"a\":1} " + "}";
         NativeObject actual = (NativeObject) parser.parseValue(json);
         assertEquals(false, actual.get("bool", actual));
         assertEquals("xyz", actual.get("str", actual));
+        assertArrayEquals(
+                "Property ordering should match",
+                new Object[] {"bool", "str", "obj"},
+                actual.getIds());
 
         NativeObject innerObj = (NativeObject) actual.get("obj", actual);
         assertEquals(1, innerObj.get("a", innerObj));
     }
 
+    @Test
+    public void testECMAKeyOrdering() throws Exception {
+        try (Context cx = Context.enter()) {
+            cx.setLanguageVersion(Context.VERSION_ES6);
+            String json =
+                    "{\"foo\": \"a\", \"bar\": \"b\", \"1\": \"c\", \"-1\": \"d\", \"x\": \"e\"}";
+            NativeObject actual = (NativeObject) parser.parseValue(json);
+            // Ensure that modern ECMAScript property ordering works, which depends on
+            // valid index values being treated as numbers and not as strings.
+            assertArrayEquals(
+                    "Property ordering should match",
+                    new Object[] {1, "foo", "bar", "-1", "x"},
+                    actual.getIds());
+        }
+    }
+
     @Test(expected = ParseException.class)
     public void shouldFailToParseJsonObjectsWithInvalidFormat() throws Exception {
         parser.parseValue("{\"only\", \"keys\"}");