diff --git a/lib/sequence.js b/lib/sequence.js
index d222194..11bdf73 100644
--- a/lib/sequence.js
+++ b/lib/sequence.js
@@ -110,13 +110,14 @@ Sequence.prototype.getId = function() {
  *                       reference
  */
 Sequence.prototype._getCounterReferenceField = function(doc) {
-    var reference = [];
+    var reference = {};
 
     if (this._useReference === false) {
         reference = null;
     }else {
         for (var i in this._options.reference_fields) {
-            reference.push(JSON.stringify(doc[this._options.reference_fields[i]]));
+            reference[this._options.reference_fields[i]] = doc[this._options.reference_fields[i]];
+            // reference.push(JSON.stringify(doc[this._options.reference_fields[i]]));
         }
     }
 
@@ -153,7 +154,7 @@ Sequence.prototype._createCounterModel = function() {
     CounterSchema = mongoose.Schema(
         {
             id: {type: String, required: true},
-            reference_value: {type:Array, required: true},
+            reference_value: { type: mongoose.Schema.Types.Mixed, required: true},
             seq: {type:Number, default: 0, required: true}
         },
         {
diff --git a/test/base.js b/test/base.js
index 709cdcb..2be2cb8 100644
--- a/test/base.js
+++ b/test/base.js
@@ -303,39 +303,39 @@ describe('Basic => ', function() {
 
         });
 
-        describe('a counter which referes others fields => ', function() {
+        describe('a counter which referes others fields => ', function () {
 
-            before(function() {
+            before(function () {
                 var ComposedSchema = new Schema({
-                    country: String,
+                    country: Schema.Types.ObjectId,
                     city: String,
                     inhabitant: Number
                 });
-                ComposedSchema.plugin(AutoIncrement, {id: 'inhabitant_counter', inc_field: 'inhabitant', reference_fields: ['country', 'city']});
+                ComposedSchema.plugin(AutoIncrement, { id: 'inhabitant_counter', inc_field: 'inhabitant', reference_fields: ['city', 'country'] });
                 this.Composed = mongoose.model('Composed', ComposedSchema);
             });
 
-            it('increment on save', function(done) {
-                var t = new this.Composed({country:'France', city:'Paris'});
-                t.save(function(err) {
+            it('increment on save', function (done) {
+                var t = new this.Composed({ country: mongoose.Types.ObjectId('59c380f51207391238e7f3f2'), city: 'Paris' });
+                t.save(function (err) {
                     if (err) return done(err);
                     assert.deepEqual(t.inhabitant, 1);
                     done();
                 });
             });
 
-            it('saving a document with the same reference increment the counter', function(done) {
-                var t = new this.Composed({country:'France', city:'Paris'});
-                t.save(function(err) {
+            it('saving a document with the same reference increment the counter', function (done) {
+                var t = new this.Composed({ country: mongoose.Types.ObjectId('59c380f51207391238e7f3f2'), city: 'Paris' });
+                t.save(function (err) {
                     if (err) return done(err);
                     assert.deepEqual(t.inhabitant, 2);
                     done();
                 });
             });
 
-            it('saving with a different reference do not increment the counter', function(done) {
-                var t = new this.Composed({country:'USA', city:'New York'});
-                t.save(function(err) {
+            it('saving with a different reference do not increment the counter', function (done) {
+                var t = new this.Composed({ country: mongoose.Types.ObjectId('59c380f51207391238e7f3f2'), city: 'Carcasonne' });
+                t.save(function (err) {
                     if (err) return done(err);
                     assert.deepEqual(t.inhabitant, 1);
                     done();