diff --git a/lib/collection.js b/lib/collection.js
index e98d967..67d5a17 100644
--- a/lib/collection.js
+++ b/lib/collection.js
@@ -29,6 +29,10 @@ function addToSet (array, other) {
   return result;
 };
 
+function validId(i) {
+  return (!!i) || i === 0 || i === "";
+}
+
 module.exports = function Collection(db, state) {
   var name = state.name;
   var pk = state.pkFactory || ObjectId;
@@ -190,7 +194,7 @@ module.exports = function Collection(db, state) {
         var insertedIds = [];
         for (var i = 0; i < docs.length; i++) {
           var doc = docs[i];
-          if(!doc._id) doc._id = pk();
+          if(!validId(doc._id)) doc._id = pk();
 
           var conflict = state.findConflict(doc);
           if(conflict) {
@@ -295,7 +299,7 @@ module.exports = function Collection(db, state) {
         if(!docs.length && options.upsert) {
           var cloneData = upsertClone(selector, data);
           var cloned = _.cloneDeepWith(cloneData, cloneObjectIDs);
-          cloned._id = selector._id || pk();
+          cloned._id = validId(selector._id) ? selector._id : pk();
 
           debug('%s.%s checking for index conflict', name, action);
           var conflict = state.findConflict(cloned);
@@ -356,7 +360,7 @@ module.exports = function Collection(db, state) {
         if(!docs.length && options.upsert) {
           var cloneData = upsertClone(selector, data);
           var cloned = _.cloneDeepWith(cloneData, cloneObjectIDs);
-          cloned._id = selector._id || pk();
+          cloned._id = validId(selector._id) ? selector._id : pk();
 
           debug('%s.%s checking for index conflict', name, action);
           var conflict = state.findConflict(cloned);
@@ -419,7 +423,7 @@ module.exports = function Collection(db, state) {
         if(!docs.length && options.upsert) {
           var cloneData = upsertClone(selector, data);
           var cloned = _.cloneDeepWith(cloneData, cloneObjectIDs);
-          cloned._id = selector._id || pk();
+          cloned._id = validId(selector._id) ? selector._id : pk();
 
           debug('%s.%s checking for index conflict', name, action);
           var conflict = state.findConflict(cloned);
diff --git a/test/mock.test.js b/test/mock.test.js
index 33e041f..1b0ad9a 100644
--- a/test/mock.test.js
+++ b/test/mock.test.js
@@ -7,7 +7,7 @@ var id = ObjectID();
 MongoClient.persist = "mongo.js";
 
 // this number is used in all the query/find tests, so it's easier to add more docs
-var EXPECTED_TOTAL_TEST_DOCS = 13;
+var EXPECTED_TOTAL_TEST_DOCS = 16;
 
 describe('mock tests', function () {
   var connected_db;
@@ -185,6 +185,40 @@ describe('mock tests', function () {
         done();
       });
     });
+    it('should allow _id to be a primitive', function (done) {
+      collection.insert({_id:15, test:457, foo:true}, function (err, result) {
+        if(err) return done(err);
+        (!!result.ops).should.be.true;
+        (!!result.ops[0]).should.be.true;
+        result.ops[0]._id.should.equal(15);
+        result.ops[0]._id.should.not.equal("15");
+        result.ops[0].should.have.property('test', 457);
+        done();
+      });
+    });
+
+    it('should allow _id to be 0', function (done) {
+      collection.insert({_id:0, test:458, foo:true}, function (err, result) {
+        if(err) return done(err);
+        (!!result.ops).should.be.true;
+        (!!result.ops[0]).should.be.true;
+        result.ops[0]._id.should.equal(0);
+        result.ops[0]._id.should.not.equal("0");
+        result.ops[0].should.have.property('test', 458);
+        done();
+      });
+    });
+
+    it('should allow _id to be ""', function (done) {
+      collection.insert({_id:"", test:459, foo:true}, function (err, result) {
+        if(err) return done(err);
+        (!!result.ops).should.be.true;
+        (!!result.ops[0]).should.be.true;
+        result.ops[0]._id.should.equal("");
+        result.ops[0].should.have.property('test', 459);
+        done();
+      });
+    });
 
     it('should findOne by a property', function (done) {
       collection.findOne({test:123}, function (err, doc) {
@@ -353,11 +387,11 @@ describe('mock tests', function () {
     it('should update multi', function (done) {
       collection.update({}, {$set:{foo:"bar"}}, {multi:true}, function (err, result) {
         if(err) return done(err);
-        result.n.should.equal(9);
+        result.n.should.equal(12);
 
         collection.find({foo:"bar"}).count(function (err, n) {
           if(err) return done(err);
-          n.should.equal(9);
+          n.should.equal(12);
           done();
         });
       });
@@ -365,14 +399,14 @@ describe('mock tests', function () {
     it('should updateMany', function (done) {
       collection.updateMany({}, {$set:{updateMany:"bar"}}, function (err, opResult) {
         if(err) return done(err);
-        opResult.result.n.should.equal(9);
-        opResult.result.nModified.should.equal(9);
-        opResult.matchedCount.should.equal(9);
-        opResult.modifiedCount.should.equal(9);
+        opResult.result.n.should.equal(12);
+        opResult.result.nModified.should.equal(12);
+        opResult.matchedCount.should.equal(12);
+        opResult.modifiedCount.should.equal(12);
 
         collection.find({updateMany:"bar"}).count(function (err, n) {
           if(err) return done(err);
-          n.should.equal(9);
+          n.should.equal(12);
           done();
         });
       });