Error "dup key: {: null}" mongodb

I have a mongoose schema like this:

const mongoose = require('mongoose');
const Schema = mongoose.Schema;
const thingsSchema = new Schema({
    userId: {
        required: true,
        unique: false,
        type: String,
    },
});
module.exports = mongoose.model('Thing', thingsSchema);

when trying to insert a new document gives me this error:

"E11000 duplicate key error index: thingsSchema.$userId_1  dup key: { : null }"

that error indicates that userId is unique, and I will insert duplicate because if it has been inserted before, but as you can see I specify unique: false . The question is that before I had unique: true , so if some were created with various userId , but then changing to not be unique I kept giving that error, even after deleting the elements of the database .
Even if you remove the userId field, you still gave that error, which is the one that showed above, that as you can see { :null } indicates that you are trying to create with value null .
I've been using mongoose and mongodb a good time, and I never had this error. I hope you can help me. Thanks in advance.