Spock error groovy.lang.MissingMethodException: when one method calls another

When running a unit test I get an error about spoc closure failing when the method being tested calls another method inside of the object under test.

The error is:

groovy.lang.MissingMethodException: No signature of method: com.mypackage.RuleRunnerServiceSpec$__spock_feature_0_9_closure12.doCall() is applicable for argument types: ([B, java.lang.Integer, java.lang.Integer) values: [[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, ...], ...]
Possible solutions: call(), doCall(java.io.InputStream), findAll()
    at com.mypackage.RuleRunnerService.loadRulesFromS3(RuleRunnerService.groovy:132)
    at com.mypackage.RuleRunnerServiceSpec.should load from s3(RuleRunnerServiceSpec.groovy:272)

The test is:

void "should load from s3"(){

        given:
            RuleRunnerService ruleService = RuleRunnerService.instance
            ruleService.grailsApplication = [
                    config: [
                            alert: [
                                    engine: [s3 : [bucketName: 'bucket']]
                            ]
                    ]
            ]
            def s3wrapper = mockFor(S3Wrapper, true)

            s3wrapper.demand.asBoolean(0..999) { -> true }
            s3wrapper.demand.getS3ObjectToInputStream(0..999){
               InputStream stream -> new FileInputStream('test/resources/samples-drl/samplefile.drl')
            }

            ruleService.s3 = s3wrapper.createMock()
        when:
            ruleService.loadRulesFromS3('test')
        then:
            ruleService.hasRulePackageByName('test')

The method being tested is:

void loadRulesFromS3(String organizationId){

        String bucketName = grailsApplication.config.alert.engine.s3.bucketName


        S3Wrapper s3 = getS3Wrapper()
        InputStream newRules = s3.getS3ObjectToInputStream(bucketName, organizationId)

        loadRulesFromString(organizationId, [newRules.text])
    }

The call exits with the error above on the loadRulesFromString(...) call. Is there some setup required to allow for internal method calls in a Spock test?