diff --git a/lib/Model.ts b/lib/Model.ts
index 6947221..a35e073 100644
--- a/lib/Model.ts
+++ b/lib/Model.ts
@@ -497,7 +497,7 @@ export class Model<TDocument extends { _id?: any }, TInstance> {
                 let docs = this._handlers.creatingDocuments(objects);
                 return docs.map((object: { _id: any; }) => {
                     return new Bluebird<any[]>((resolve, reject) => {
-                        this.collection.findOneAndUpdate({ _id: object._id }, object, {
+                        this.collection.findOneAndUpdate({ _id: object._id || { $exists: false }}, object, {
                             upsert: options.upsert,
                             returnOriginal: false
                         }, (err, result) => {
diff --git a/test/Model.ts b/test/Model.ts
index 51a6b22..56b3345 100644
--- a/test/Model.ts
+++ b/test/Model.ts
@@ -217,6 +217,10 @@ describe("Model",() => {
             return chai.expect(model.create({ answer: 14 }, { upsert: true })).to.eventually.exist;
         });
 
+        it("should generate an _id for new, upserted, documents", () => {
+            return chai.expect(model.create({ answer: 12 }, { upsert: true })).to.eventually.exist.and.have.property("_id").and.exist;
+        })
+
         it("should return an error if you don't meet the schema validation requirements",() => {
            return chai.expect(model.create(<any>{ answer: "wrong" })).to.eventually.be.rejected;
         });
@@ -265,6 +269,10 @@ describe("Model",() => {
             return chai.expect(model.insert({ answer: 14 }, { upsert: true })).to.eventually.exist;
         });
 
+        it("should generate an _id for new, upserted, documents", () => {
+            return chai.expect(model.insert({ answer: 12 }, { upsert: true })).to.eventually.exist.and.have.property("_id").and.exist;
+        })
+
         it("should return an error if you don't meet the schema validation requirements",() => {
            return chai.expect(model.insert(<any>{ answer: "wrong" })).to.eventually.be.rejected;
         });