diff --git a/src/org/mozilla/javascript/NativeArray.java b/src/org/mozilla/javascript/NativeArray.java
index 9096dec92e..e861e809e2 100644
--- a/src/org/mozilla/javascript/NativeArray.java
+++ b/src/org/mozilla/javascript/NativeArray.java
@@ -481,6 +481,14 @@ public Object execIdCall(
         }
     }
 
+    @Override
+    public void setPrototype(Scriptable p) {
+        super.setPrototype(p);
+        if (!(p instanceof NativeArray)) {
+            setDenseOnly(false);
+        }
+    }
+
     @Override
     public Object get(int index, Scriptable start) {
         if (!denseOnly && isGetterOrSetter(null, index, false)) return super.get(index, start);
diff --git a/testsrc/jstests/es6/array-concat.js b/testsrc/jstests/es6/array-concat.js
new file mode 100644
index 0000000000..ddc129c431
--- /dev/null
+++ b/testsrc/jstests/es6/array-concat.js
@@ -0,0 +1,159 @@
+/*
+ * This is a set of tests for array concatenation in ES6.
+ */
+'use strict'; 
+
+load("testsrc/assert.js");
+
+function makeArrayLike(len, concatSpreadable) {
+  var al = {};
+  for (var i = 0; i < len; i++) {
+    al[i] = i + 1;
+  }
+  al.length = len;
+  if (concatSpreadable) {
+    al[Symbol.isConcatSpreadable] = true;
+  }
+  return al;
+}
+
+function makeArrayObject(len) {
+  var al = new Array(len);
+  for (var i = 0; i < len; i++) {
+    al[i] = i + 1;
+  }
+  return al;
+}
+
+function makeJavaArray(len) {
+  var ja = java.lang.reflect.Array.newInstance(java.lang.Integer, len);
+  for (var i = 0; i < len; i++) {
+    ja[i] = i + 1;
+  }
+  return ja;
+}
+
+// Existing test case, now being run in a different way
+var a = ['a0', 'a1'];
+a[3] = 'a3';
+var b = ['b1', 'b2'];
+var cr = b.concat(a);
+assertArrayEquals(['b1', 'b2', 'a0', 'a1', undefined, 'a3'], cr);
+
+// Two native arrays
+cr = [1, 2, 3].concat([4, 5, 6]);
+assertArrayEquals([1, 2, 3, 4, 5, 6], cr);
+
+// Native array plus individual elements
+cr = [1, 2, 3].concat('four', 'five');
+assertArrayEquals([1, 2, 3, 'four', 'five'], cr);
+
+// Native array plus object
+var no = {
+  foo: 'bar'
+};
+cr = ['foo'].concat(no);
+assertArrayEquals(['foo', no], cr);
+
+// Native array plus array-like object -- do not concat
+var al = makeArrayLike(4);
+cr = [1, 2, 3].concat(al);
+assertArrayEquals([1, 2, 3, al], cr);
+
+// Array-like object plus native array -- do not concat
+al = makeArrayLike(2);
+cr = Array.prototype.concat.call(al, [3, 4]);
+assertArrayEquals([al, 3, 4], cr);
+
+// Non-array plus native array
+cr = Array.prototype.concat.call('one', [2, 3]);
+// ToObject conversion makes this comparision different
+assertEquals(3, cr.length);
+assertEquals('object', typeof cr[0]);
+assertTrue('one' == cr[0]);
+assertEquals(2, cr[1]);
+assertEquals(3, cr[2]);
+
+// Three non-arrays
+cr = Array.prototype.concat.call('one', 'two', 'three');
+assertEquals(3, cr.length);
+assertEquals('object', typeof cr[0]);
+assertTrue('one' == cr[0]);
+assertEquals('two', cr[1]);
+assertEquals('three', cr[2]);
+
+// Array-liks objects with a @isConcatSpreadable symbol
+cr = Array.prototype.concat.call(makeArrayLike(2, true));
+assertArrayEquals([1, 2], cr);
+cr = Array.prototype.concat.call(makeArrayLike(2, true), [3, 4]);
+assertArrayEquals([1, 2, 3, 4], cr);
+cr = Array.prototype.concat.call(makeArrayLike(2, true), makeArrayLike(2, true));
+assertArrayEquals([1, 2, 1, 2], cr);
+cr = Array.prototype.concat.call(makeArrayLike(2, true), 1, 2);
+assertArrayEquals([1, 2, 1, 2], cr);
+cr = [1, 2].concat(makeArrayLike(2, true));
+assertArrayEquals([1, 2, 1, 2], cr);
+
+// Native array plus Java array. Java arrays are "arrays" for the purposes of this
+// function, and this is how things functioned in previous Rhino releases.
+cr = [1, 2].concat(makeJavaArray(2));
+// Java object conversion makes array comparision harder
+assertEquals(4, cr.length);
+assertTrue(cr[0] == 1);
+assertTrue(cr[1] == 2);
+assertTrue(cr[2] == 1);
+assertTrue(cr[3] == 2);
+
+// Java array plus native array
+cr = Array.prototype.concat.call(makeJavaArray(2), [3, 4]);
+assertEquals(4, cr.length);
+assertTrue(cr[0] == 1);
+assertTrue(cr[1] == 2);
+assertTrue(cr[2] == 3);
+assertTrue(cr[3] == 4);
+
+// Two java arrays
+cr = Array.prototype.concat.call(makeJavaArray(2), makeJavaArray(3));
+assertEquals(5, cr.length);
+assertTrue(cr[0] == 1);
+assertTrue(cr[1] == 2);
+assertTrue(cr[2] == 1);
+assertTrue(cr[3] == 2);
+assertTrue(cr[4] == 3);
+
+// A few operations on the Array object
+cr = new Array(3).concat();
+assertArrayEquals([undefined, undefined, undefined], cr);
+cr = makeArrayObject(3).concat();
+assertArrayEquals([1, 2, 3], cr);
+cr = makeArrayObject(3).concat([4, 5, 6]);
+assertArrayEquals([1, 2, 3, 4, 5, 6], cr);
+cr = makeArrayObject(1).concat(makeArrayLike(2, false));
+assertEquals(2, cr.length);
+assertEquals(1, cr[0]);
+assertEquals('object', typeof cr[1]);
+cr = makeArrayObject(3).concat(makeArrayLike(2, true));
+assertArrayEquals([1, 2, 3, 1, 2], cr);
+
+// Ensure that the Array object lets us change the prototype
+let getterCalled = false;
+let proto = {};
+Object.defineProperty(proto, 0, {
+  get: () => {
+    getterCalled = true;
+    return 'z';
+  }
+});
+
+a = new Array(3);
+assertEquals(undefined, a[0]);
+Object.setPrototypeOf(a, proto);
+assertEquals('z', a[0]);
+assertTrue(getterCalled);
+getterCalled = false;
+cr = Array.prototype.concat.call(a);
+assertArrayEquals(['z', undefined, undefined], cr);
+assertTrue(getterCalled);
+
+'success';
+
diff --git a/testsrc/org/mozilla/javascript/tests/es6/ArrayConcat.java b/testsrc/org/mozilla/javascript/tests/es6/ArrayConcat.java
new file mode 100644
index 0000000000..bf52d1bc3a
--- /dev/null
+++ b/testsrc/org/mozilla/javascript/tests/es6/ArrayConcat.java
@@ -0,0 +1,10 @@
+package org.mozilla.javascript.tests.es6;
+
+import org.mozilla.javascript.Context;
+import org.mozilla.javascript.drivers.LanguageVersion;
+import org.mozilla.javascript.drivers.RhinoTest;
+import org.mozilla.javascript.drivers.ScriptTestsBase;
+
+@RhinoTest("testsrc/jstests/es6/array-concat.js")
+@LanguageVersion(Context.VERSION_ES6)
+public class ArrayConcat extends ScriptTestsBase {}