Fail/Error in cordapp-template FlowTest progressTracker hasn't been started

when I try to run this tests for the flows in the cordapp-template:

@Test
fun flowRecordTransactionInBothVaults() {

    val flow = IOUFlow.Initiator(1,b.info.legalIdentity)
    val future = a.services.startFlow(flow).resultFuture
    net.runNetwork()
    val signedTx = future.getOrThrow()

    for (node in listOf(a,b)) {
        assertEquals(signedTx, node.storage.validatedTransactions.getTransaction(signedTx.id))
    }
}

I get this error: Progress tracker hasn't been started

[INFO ] 15:14:52.144 [Mock network] AbstractNetworkMapService.processRegistrationRequest - Added node CN=Mock Company 3,O=R3,L=New York,C=US to network map

[WARN ] 15:14:52.172 [Mock network] [a11087fc-381d-4547-8736-5265c334c71f].maybeWireUpProgressTracking - ProgressTracker has not been started

[WARN ] 15:14:52.191 [Mock network] [0dcfa270-b1af-40e9-92f1-411334cf0c73].run - Terminated by unexpected exceptionkotlin.NotImplementedError: An operation is not implemented: not implemented
at com.template.flow.IOUFlow$Acceptor$call$1.checkTransaction(TemplateFlow.kt:225) ~[main/:?] 

at net.corda.flows.SignTransactionFlow.call(CollectSignaturesFlow.kt:201) ~[corda-core-0.13.0.jar:?]
at net.corda.flows.SignTransactionFlow.call(CollectSignaturesFlow.kt:177) ~[corda-core-0.13.0.jar:?]

[WARN ] 15:14:52.202 [Mock network] [a11087fc-381d-4547-8736-5265c334c71f].run - Terminated by unexpected exceptionnet.corda.core.flows.FlowSessionException: Counterparty flow on CN=Mock Company 3,O=R3,L=New York,C=US had an internal error and has terminated
at net.corda.node.services.statemachine.FlowStateMachineImpl.erroredEnd(FlowStateMachineImpl.kt:382) ~[corda-node

[WARN ] 15:14:52.203 [Mock network] [a11087fc-381d-4547-8736-5265c334c71f].uncaughtException - Caught exception from flowjava.lang.IllegalStateException: Progress tracker has already ended
at net.corda.core.utilities.ProgressTracker.endWithError

Actually the code is much longer but I think these are the relevant parts. Is it a known thing? How can I fix it?