Geb: how to register unexpexted pages

I registered a page as unexpected like below in GebConfig.groovy:

unexpectedPages = [EmptySearchResultPage]

and the page class is like below:

import geb.Page

class EmptySearchResultPage extends Page{
    static at = {
//      errorTitle.displayed
         $("h1",text:contains('Sorry… We Found No Flights.')) 
        }
    static content = {
        errorTitle(wait:true,required:false) { $("h1",text:contains('Sorry… We Found No XXX.')) }
    }
}

But I am getting following exception:

geb.error.InvalidGebConfiguration: Unexpected pages configuration has to be a collection of classes that extend geb.Page but found "[[:]]". Did you forget to include some imports in your config file?
    at geb.Configuration.getUnexpectedPages(Configuration.groovy:189)
    at geb.Browser.doCheckIfAtAnUnexpectedPage(Browser.groovy:416)
    at geb.Browser.checkIfAtAnUnexpectedPage(Browser.groovy:402)
    at geb.Page.verifyAt(Page.groovy:181)
    at geb.Browser.doAt(Browser.groovy:454)
    at geb.Browser.at(Browser.groovy:353)
    at geb.Browser.to(Browser.groovy:568)
    at geb.Browser.to(Browser.groovy:556)
    at geb.Browser.to(Browser.groovy:523)
    at geb.spock.GebSpec.methodMissing(GebSpec.groovy:56)
    at com.skygate.global.HomePageTest.Open Homepage(HomePageTest.groovy:59)

What is the mistake I made? Please help. Thank you.