Devise not working in production due to nil to_sym method

Question

I have been trying to add user login / authentication to my RoR site. I am using Devise.

Things work fine in my dev instance (I use Cloud9, in case useful).

However when I push to Heroku / production, for any page related to the user functionality (e.g. User Sign up, User login, or a page which checks if the user is logged in), it errors out. All Heroku gives me is:

NoMethodError (undefined method `to_sym' for nil:NilClass):

Just looking for some ideas on what could be happening?

user.rb:

class User < ApplicationRecord
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable, :trackable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :validatable
         
  enum role: [:user, :business, :admin]
  after_initialize :set_default_role, :if => :new_record?
  def set_default_role
    self.role ||= :user
  end

DeviseCreateUsers migration:

# frozen_string_literal: true

class DeviseCreateUsers < ActiveRecord::Migration[5.0]
  def change
    create_table :users do |t|
      ## Database authenticatable
      t.string :email,              null: false, default: ""
      t.string :encrypted_password, null: false, default: ""

      ## Recoverable
      t.string   :reset_password_token
      t.datetime :reset_password_sent_at

      ## Rememberable
      t.datetime :remember_created_at

      ## Trackable
      # t.integer  :sign_in_count, default: 0, null: false
      # t.datetime :current_sign_in_at
      # t.datetime :last_sign_in_at
      # t.string   :current_sign_in_ip
      # t.string   :last_sign_in_ip

      ## Confirmable
      # t.string   :confirmation_token
      # t.datetime :confirmed_at
      # t.datetime :confirmation_sent_at
      # t.string   :unconfirmed_email # Only if using reconfirmable

      ## Lockable
      # t.integer  :failed_attempts, default: 0, null: false # Only if lock strategy is :failed_attempts
      # t.string   :unlock_token # Only if unlock strategy is :email or :both
      # t.datetime :locked_at
      
      t.integer :role, default: 0

      t.timestamps null: false
    end

    add_index :users, :email,                unique: true
    add_index :users, :reset_password_token, unique: true
    # add_index :users, :confirmation_token,   unique: true
    # add_index :users, :unlock_token,         unique: true
  end
end

Many thanks!