Authlogic - no implicit conversion of nil into Integer

Question

I'm updating an old Rails (Ruby v 2.5.6 & Rails v 4.2.10) app and I've hit a weird error where when a user attempts to log in, if the password is incorrect I get

no implicit conversion of nil into Integer

on the line with

if @user_session.save

All the code is stock from authlogic so I can't understand why it's occurring

class UserSessionsController < BaseGUIController
  include CurrentCart

  def new
    @user_session = UserSession.new
  end

  def create
    @user_session = UserSession.new(user_session_params)
    if @user_session.save
      flash[:success] = "Welcome back!"
      redirect_to root_url
    else
      render :new
    end
  end

  private

  def user_session_params
    params.require(:user_session).permit(:email, :password, :remember_me)
  end
end

It should return @user_session as not saved and redirect the user back to the login for to try again. If the password is correct there is no issue.

Am I missing something really obvious?

Thank you.