diff --git a/lib/chai/core/assertions.js b/lib/chai/core/assertions.js
index 349e53ddc..4a4d2d28f 100644
--- a/lib/chai/core/assertions.js
+++ b/lib/chai/core/assertions.js
@@ -1067,7 +1067,7 @@ module.exports = function (chai, _) {
         ok
       , 'expected #{this} to ' + str
       , 'expected #{this} to not ' + str
-      , expected.sort()
+      , expected.slice(0).sort()
       , actual.sort()
       , true
     );
diff --git a/test/expect.js b/test/expect.js
index bd5dcc7ec..acb02dd07 100644
--- a/test/expect.js
+++ b/test/expect.js
@@ -668,6 +668,15 @@ describe('expect', function () {
 
   });
 
+  it('keys(array) will not mutate array (#359)', function () {
+      var expected = [ 'b', 'a' ];
+      var original_order = [ 'b', 'a' ];
+      var obj = { "b": 1, "a": 1 };
+      expect(expected).deep.equal(original_order);
+      expect(obj).keys(original_order);
+      expect(expected).deep.equal(original_order);
+  });
+
   it('chaining', function(){
     var tea = { name: 'chai', extras: ['milk', 'sugar', 'smile'] };
     expect(tea).to.have.property('extras').with.lengthOf(3);
diff --git a/test/should.js b/test/should.js
index c122a01a6..d28861f06 100644
--- a/test/should.js
+++ b/test/should.js
@@ -525,6 +525,15 @@ describe('should', function() {
     }, "expected { foo: 1, bar: 2 } to not have keys 'foo', or 'baz'");
   });
 
+  it('keys(array) will not mutate array (#359)', function () {
+      var expected = [ 'b', 'a' ];
+      var original_order = [ 'b', 'a' ];
+      var obj = { "b": 1, "a": 1 };
+      expected.should.deep.equal(original_order);
+      obj.should.keys(original_order);
+      expected.should.deep.equal(original_order);
+  });
+
   it('throw', function () {
     // See GH-45: some poorly-constructed custom errors don't have useful names
     // on either their constructor or their constructor prototype, but instead