diff --git a/lib/chai/core/assertions.js b/lib/chai/core/assertions.js
index 1600b2c98..069c5241b 100644
--- a/lib/chai/core/assertions.js
+++ b/lib/chai/core/assertions.js
@@ -1142,6 +1142,7 @@ module.exports = function (chai, _) {
       , ssfi = flag(this, 'ssfi')
       , objType = _.type(obj).toLowerCase()
       , nType = _.type(n).toLowerCase()
+      , errorMessage
       , shouldThrow = true;
 
     if (doLength) {
@@ -1238,6 +1239,7 @@ module.exports = function (chai, _) {
       , ssfi = flag(this, 'ssfi')
       , objType = _.type(obj).toLowerCase()
       , nType = _.type(n).toLowerCase()
+      , errorMessage
       , shouldThrow = true;
 
     if (doLength) {
@@ -1333,6 +1335,7 @@ module.exports = function (chai, _) {
       , ssfi = flag(this, 'ssfi')
       , objType = _.type(obj).toLowerCase()
       , nType = _.type(n).toLowerCase()
+      , errorMessage
       , shouldThrow = true;
 
     if (doLength) {
@@ -1428,6 +1431,7 @@ module.exports = function (chai, _) {
       , ssfi = flag(this, 'ssfi')
       , objType = _.type(obj).toLowerCase()
       , nType = _.type(n).toLowerCase()
+      , errorMessage
       , shouldThrow = true;
 
     if (doLength) {
@@ -1524,6 +1528,7 @@ module.exports = function (chai, _) {
       , objType = _.type(obj).toLowerCase()
       , startType = _.type(start).toLowerCase()
       , finishType = _.type(finish).toLowerCase()
+      , errorMessage
       , shouldThrow = true
       , range = (startType === 'date' && finishType === 'date')
           ? start.toUTCString() + '..' + finish.toUTCString()
diff --git a/test/assert.js b/test/assert.js
index ab12e2921..e3483ee97 100644
--- a/test/assert.js
+++ b/test/assert.js
@@ -180,19 +180,12 @@ describe('assert', function () {
       assert.instanceOf(new Foo(), undefined);
     }, "The instanceof assertion needs a constructor but undefined was given.");
 
-    var expectedError;
-    try {
-      t instanceof Thing;
-    } catch (err) {
-      errMsg = '[object Object] instanceof function Thing(){} failed: ' + err.message + '.';
-    }
-
     err(function(){
       function Thing(){};
       var t = new Thing();
       Thing.prototype = 1337;
       assert.instanceOf(t, Thing);
-    }, expectedError, true);
+    }, 'The instanceof assertion needs a constructor but function was given.', true);
 
     if (typeof Symbol !== 'undefined' && typeof Symbol.hasInstance !== 'undefined') {
         err(function(){
diff --git a/test/expect.js b/test/expect.js
index 043c734e5..0b5a0e5b2 100644
--- a/test/expect.js
+++ b/test/expect.js
@@ -413,20 +413,12 @@ describe('expect', function () {
       expect(new Foo()).to.an.instanceof(undefined);
     }, "The instanceof assertion needs a constructor but undefined was given.");
 
-    // Different browsers may have different error messages
-    var expectedError;
-    try {
-      t instanceof Thing;
-    } catch (err) {
-      errMsg = '[object Object] instanceof function Thing(){} failed: ' + err.message + '.';
-    }
-
     err(function(){
       function Thing(){};
       var t = new Thing();
       Thing.prototype = 1337;
       expect(t).to.an.instanceof(Thing);
-    }, expectedError, true)
+    }, 'The instanceof assertion needs a constructor but function was given.', true)
 
     if (typeof Symbol !== 'undefined' && typeof Symbol.hasInstance !== 'undefined') {
         err(function(){
diff --git a/test/should.js b/test/should.js
index 4585bfc1b..1453512d6 100644
--- a/test/should.js
+++ b/test/should.js
@@ -455,20 +455,12 @@ describe('should', function() {
       new Foo().should.be.an.instanceof(undefined);
     }, "The instanceof assertion needs a constructor but undefined was given.");
 
-    // Different browsers may have different error messages
-    var expectedError;
-    try {
-      t instanceof Thing;
-    } catch (err) {
-      errMsg = '[object Object] instanceof function Thing(){} failed: ' + err.message + '.';
-    }
-
     err(function(){
       function Thing(){};
       var t = new Thing();
       Thing.prototype = 1337;
       t.should.be.an.instanceof(Thing);
-    }, expectedError, true);
+    }, 'The instanceof assertion needs a constructor but function was given.', true);
 
     if (typeof Symbol !== 'undefined' && typeof Symbol.hasInstance !== 'undefined') {
         err(function(){
diff --git a/test/utilities.js b/test/utilities.js
index 58b43c48e..50963f2f1 100644
--- a/test/utilities.js
+++ b/test/utilities.js
@@ -132,8 +132,8 @@ describe('utilities', function () {
     });
 
     it('addMethod returns new assertion with flags copied over', function () {
-      assertion1 = expect('foo');
-      assertion2 = assertion1.to.returnNewAssertion();
+      var assertion1 = expect('foo');
+      var assertion2 = assertion1.to.returnNewAssertion();
 
       // Checking if a new assertion was returned
       expect(assertion1).to.not.be.equal(assertion2);
@@ -413,8 +413,8 @@ describe('utilities', function () {
     });
 
     it('addProperty returns a new assertion with flags copied over', function () {
-      assertion1 = expect('foo');
-      assertion2 = assertion1.is.thing;
+      var assertion1 = expect('foo');
+      var assertion2 = assertion1.is.thing;
 
       // Checking if a new assertion was returned
       expect(assertion1).to.not.be.equal(assertion2);
@@ -829,8 +829,9 @@ describe('utilities', function () {
             new chai.Assertion(this._obj).to.be.equal('x');
           }
         , function () {
-            this._obj = this._obj || {};
-            this._obj.__x = 'X!'
+            if (this._obj === Object(this._obj)) {
+              this._obj.__x = 'X!'
+            }
           }
         );
 
@@ -939,8 +940,9 @@ describe('utilities', function () {
             new chai.Assertion(this._obj).to.be.equal('x');
           }
         , function () {
-            this._obj = this._obj || {};
-            this._obj.__x = 'X!'
+            if (this._obj === Object(this._obj)) {
+              this._obj.__x = 'X!'
+            }
           }
         );