Spock framework VerifyError: (class: BuggedTest, method: $spock_feature_0_0 signature: ()V) Stack size too large bug

bug.groovy

@Grab("org.spockframework:spock-core:0.7-groovy-2.0")

import spock.lang.Specification

class BuggedTest extends Specification {

	def thing = []

	def "repo the bug"() {
		given:
		thing.add([helperMethod()])

		when:
		thing.toString()

		then:
		thrown(AssertionError)
	}

	def helperMethod() {
		null
	}
}