diff --git a/test/types/docArray.test.ts b/test/types/docArray.test.ts
index c296ce6fea7..78650845ba2 100644
--- a/test/types/docArray.test.ts
+++ b/test/types/docArray.test.ts
@@ -91,7 +91,7 @@ async function gh13424() {
   const TestModel = model('Test', testSchema);
 
   const doc = new TestModel();
-  expectType<Types.ObjectId | undefined>(doc.subDocArray[0]._id);
+  expectType<Types.ObjectId>(doc.subDocArray[0]._id);
 }
 
 async function gh14367() {
diff --git a/test/types/schema.test.ts b/test/types/schema.test.ts
index 9dab60d2022..49f0f277a4c 100644
--- a/test/types/schema.test.ts
+++ b/test/types/schema.test.ts
@@ -1386,7 +1386,7 @@ function gh13424() {
   const TestModel = model('TestModel', new Schema(testSchema));
 
   const doc = new TestModel({});
-  expectType<Types.ObjectId | undefined>(doc.subDocArray[0]._id);
+  expectType<Types.ObjectId>(doc.subDocArray[0]._id);
 }
 
 function gh14147() {
diff --git a/types/document.d.ts b/types/document.d.ts
index c0fb5589240..93d1718fea9 100644
--- a/types/document.d.ts
+++ b/types/document.d.ts
@@ -22,7 +22,7 @@ declare module 'mongoose' {
     constructor(doc?: any);
 
     /** This documents _id. */
-    _id?: T;
+    _id: T;
 
     /** This documents __v. */
     __v?: any;