Declarative client with query parameters resulting in "no implementation found for this method"

[olliefreeman]

I have the following client interface

@Client( "youtrack")
@Produces(APPLICATION_JSON)
@Consumes(APPLICATION_JSON)
@Requires(YoutrackClientConfiguration.PREFIX + ".authToken")
public interface YoutrackClient {

    @Get("issues")
    List<Issue> issues(@QueryValue @NotBlank String query, @QueryValue @Format("multi") Iterable<String> customFields);
...
}

and service method calling this

 public List<Issue> listIssues(TicketType ticketType) {
        String query = null;
        List<String> customFields = new ArrayList<>(BASE_CUSTOM_FIELDS);
        ...
        return query != null ? youtrackClient.issues(query, customFields) : Collections.emptyList();
    }

under test code this works perfectly and generates urls exactly as expected, and makes the calls off the test server and gets responses, all works fine.

however when running the application I get the following exception error

13:53:50.851 [main] ERROR io.micronaut.runtime.Micronaut - Error starting Micronaut server: All possible Introduction advise exhausted and no implementation found for method: List issues(String query,Iterable<String T> customFields)
io.micronaut.aop.exceptions.UnimplementedAdviceException: All possible Introduction advise exhausted and no implementation found for method: List issues(String query,Iterable<String T> customFields)
	at io.micronaut.aop.chain.MethodInterceptorChain.proceed(MethodInterceptorChain.java:132)
	at io.micronaut.validation.ValidatingInterceptor.validateReturnMicronautValidator(ValidatingInterceptor.java:153)
	at io.micronaut.validation.ValidatingInterceptor.intercept(ValidatingInterceptor.java:139)
	at io.micronaut.aop.chain.MethodInterceptorChain.proceed(MethodInterceptorChain.java:143)
	at YoutrackClient$Intercepted.issues(Unknown Source)
	at YoutrackService.listIssues(YoutrackService.java:57)

can anyone help explain, either what am I missing in the non-testruntime, or why on earth this doesn't work??

I know can fallback on the low-level client, but I'd rather not as the declarative is so much neater