Grails- Web Flow Spring- redirection error in unit test

I want to test my web flow in the grails-app in unit-test. I got redirection error.

the code of the web-flow is:

def redesignSignup = {
        redirect(action: "signupRedesign")
    }

//  Responsible for the new signup flow
    def signupRedesignFlow = {
        start {
            on("yes").to"createAccount"
        }
        createAccount{
            on("next"){
                flow.step = 2
                flash.name = "flash"}.to"congrats"
            on("back"){}.to "start"
        }

        congrats {
            redirect(controller: "connection", action: "details", params: [newUser: true])
        }

    }

he code of the unit-test is:

class WizardControllerSpec extends Specification {

    def "test next"(){
//        step1->step2
        when:"user click yes in start"
        then:"next page is create account"
        assert "createAccount" == signupRedesignFlow.start.on.yes.to

        when:"user click next in createAccount"
        then:"next page is connectPeople"
        assert "connectPeople" == signupRedesignFlow.createAccount.on.next.to


        when:"user click next in invitePartners"
        then:"next page is congrats"
        assert "congrats" == signupRedesignFlow.invitePartners.on.next.to

    }

The error I get is: org.codehaus.groovy.grails.web.servlet.mvc.exceptions.CannotRedirectException: Cannot issue a redirect(..) here. A previous call to redirect(..)

**noticed that there is no test that even call the redirect in the congrats