assertion_handler: undefined method `[]=' for nil:NilClass (NoMethodError)

[eni9889]

oauth.assertion_handler['facebook.com'] = lambda do |client, assertion, requested_scope|
    facebook = URI.parse('https://graph.facebook.com/me?access_token=' + assertion)
    response = Net::HTTP.get_response(facebook)
    puts response
end

leads to the error: undefined method `[]=' for nil:NilClass (NoMethodError)