diff --git a/mongo/bulk_write.go b/mongo/bulk_write.go
index a7efd551e7..f054c1e7ac 100644
--- a/mongo/bulk_write.go
+++ b/mongo/bulk_write.go
@@ -171,7 +171,7 @@ func (bw *bulkWrite) runInsert(ctx context.Context, batch bulkWriteBatch) (opera
 		if err != nil {
 			return operation.InsertResult{}, err
 		}
-		doc, _, err = ensureID(doc, primitive.NewObjectID(), bw.collection.bsonOpts, bw.collection.registry)
+		doc, _, err = ensureID(doc, primitive.NilObjectID, bw.collection.bsonOpts, bw.collection.registry)
 		if err != nil {
 			return operation.InsertResult{}, err
 		}
diff --git a/mongo/collection.go b/mongo/collection.go
index ac173307ff..c7b2a8a113 100644
--- a/mongo/collection.go
+++ b/mongo/collection.go
@@ -256,7 +256,7 @@ func (coll *Collection) insert(ctx context.Context, documents []interface{},
 		if err != nil {
 			return nil, err
 		}
-		bsoncoreDoc, id, err := ensureID(bsoncoreDoc, primitive.NewObjectID(), coll.bsonOpts, coll.registry)
+		bsoncoreDoc, id, err := ensureID(bsoncoreDoc, primitive.NilObjectID, coll.bsonOpts, coll.registry)
 		if err != nil {
 			return nil, err
 		}
diff --git a/mongo/mongo.go b/mongo/mongo.go
index 393c5b7713..47746ffd30 100644
--- a/mongo/mongo.go
+++ b/mongo/mongo.go
@@ -219,6 +219,9 @@ func ensureID(
 	const extraSpace = 17
 	doc = make(bsoncore.Document, 0, len(olddoc)+extraSpace)
 	_, doc = bsoncore.ReserveLength(doc)
+	if oid.IsZero() {
+		oid = primitive.NewObjectID()
+	}
 	doc = bsoncore.AppendObjectIDElement(doc, "_id", oid)
 
 	// Remove and re-write the BSON document length header.