issue with engine and adding to gemspec

[ljonny18]

I am using If I include the gem(s) in the Gemfile everything works as expected:
gem 'devise', '4.2.0'
gem 'devise_ldap_authenticatable', '0.8.5'

... however, if I include the gem(s) in my engines .gemspec file:
Gem::Specification.new do |s|
...
s.add_dependency "devise", "4.2.0"
s.add_dependency "devise_ldap_authenticatable", "0.8.5"
...
end

I get the following exception:

Exiting
/Users/jwilliam/is_web_engine/config/initializers/devise.rb:6:in block in <top (required)>': undefined method ldap_create_user=' for Devise:Module (NoMethodError)

What am I doing wrong?

Thanks,
Jon.