Mongoose: validation error path is required

In response to your last comment.

You are correct that null is a value type, but null types are a way of telling the interpreter that it has no value. therefore, you must set the values to any non-null value or you get the error. in your case set those values to empty Strings. i.e.

var newUser = new user({

  /* We will set the username, email and password field to null because they will be set later. */
  username: '',
  passwordHash: '',
  email: '',
  admin: false

}, { _id: false });