diff --git a/lib/model.js b/lib/model.js
index 8d618d0..ba85dba 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -409,12 +409,13 @@ function modify (obj, updateQuery) {
 
       if (!modifierFunctions[m]) { throw "Unknown modifier " + m; }
 
-      try {
-        keys = Object.keys(updateQuery[m]);
-      } catch (e) {
+      // Can't rely on Object.keys throwing on non objects since ES6{
+      // Not 100% satisfying as non objects can be interpreted as objects but no false negatives so we can live with it
+      if (typeof updateQuery[m] !== 'object') {
         throw "Modifier " + m + "'s argument must be an object";
       }
 
+      keys = Object.keys(updateQuery[m]);
       keys.forEach(function (k) {
         modifierFunctions[m](newDoc, k, updateQuery[m][k]);
       });
diff --git a/package.json b/package.json
index 5563164..efd65c3 100644
--- a/package.json
+++ b/package.json
@@ -21,12 +21,12 @@
   },
   "dependencies": {
     "async": "0.2.10",
-    "underscore": "~1.4.4",
     "binary-search-tree": "0.2.4",
-    "mkdirp": "~0.5.1"
+    "mkdirp": "~0.5.1",
+    "underscore": "~1.4.4"
   },
   "devDependencies": {
-    "chai": "1.0.x",
+    "chai": "^3.2.0",
     "mocha": "1.4.x",
     "request": "2.9.x",
     "sinon": "1.3.x",
diff --git a/test/model.test.js b/test/model.test.js
index 0ab78c1..7e6a3cd 100644
--- a/test/model.test.js
+++ b/test/model.test.js
@@ -1,6 +1,7 @@
 var model = require('../lib/model')
   , should = require('chai').should()
   , assert = require('chai').assert
+  , expect = require('chai').expect
   , _ = require('underscore')
   , async = require('async')
   , util = require('util')
@@ -309,9 +310,9 @@ describe('Model', function () {
         , updateQuery = { replace: 'done', bloup: [ 1, 8], _id: 'donttryit' }
         ;
 
-      (function () {
+      expect(function () {
         model.modify(obj, updateQuery);
-      }).should.throw();
+      }).to.throw("You cannot change a document's _id");
 
       updateQuery._id = 'keepit';
       model.modify(obj, updateQuery);   // No error thrown
@@ -321,27 +322,27 @@ describe('Model', function () {
       var obj = { some: 'thing' }
         , updateQuery = { replace: 'me', $modify: 'metoo' };
 
-      (function () {
+      expect(function () {
         model.modify(obj, updateQuery);
-      }).should.throw();
+      }).to.throw("You cannot mix modifiers and normal fields");
     });
 
     it('Throw an error if trying to use an inexistent modifier', function () {
       var obj = { some: 'thing' }
-        , updateQuery = { $set: 'this exists', $modify: 'not this one' };
+        , updateQuery = { $set: { it: 'exists' }, $modify: 'not this one' };
 
-      (function () {
+      expect(function () {
         model.modify(obj, updateQuery);
-      }).should.throw();
+      }).to.throw(/^Unknown modifier .modify/);
     });
 
     it('Throw an error if a modifier is used with a non-object argument', function () {
       var obj = { some: 'thing' }
         , updateQuery = { $set: 'this exists' };
 
-      (function () {
+      expect(function () {
         model.modify(obj, updateQuery);
-      }).should.throw();
+      }).to.throw(/Modifier .set's argument must be an object/);
     });
 
     describe('$set modifier', function () {