无法将新对象添加到文档

I am trying to add a new field productFiles(along with its field) to an existing document in MongoDb. But I am getting follwing error:

"the positional operator did not find the match needed from the query."

query := bson.M{
     "productId": productId,
}

update := bson.M{
    "$set": bson.M{ 
        "productFiles.$.updatedOn": int32(time.Now().Unix()),
        "productFiles.$.updatedBy": userId,
    },
}

err := repo.Collection().Update(query, update)