Spring Data REST Custom Resource URI works for String but not Long

I have a model:

public class MyModel {
    @Id private Long id;
    private Long externalId;
    // Getters, setters
}

I'd like to use externalId as my resource identifier:

@Configuration
static class RepositoryEntityLookupConfig extends RepositoryRestConfigurerAdapter {
    @Override
    public void configureRepositoryRestConfiguration(RepositoryRestConfiguration configuration) {
        configuration
                .withEntityLookup()
                    .forRepository(MyRepository.class, MyModel::getExternalId, MyRepository::findByExternalId);
    }
}

If externalId is a String, this works fine. But since it's a number (Long)

public interface MyRepository extends JpaRepository<MyModel, Long> {
    Optional<MyModel> findByExternalId(@Param("externalId") Long externalId);
}

when invoking: /myModels/1 I get:

java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long
    at org.springframework.data.rest.core.config.EntityLookupConfiguration$RepositoriesEntityLookup.lookupEntity(EntityLookupConfiguration.java:213) ~[spring-data-rest-core-2.6.4.RELEASE.jar:na]
    at org.springframework.data.rest.core.support.UnwrappingRepositoryInvokerFactory$UnwrappingRepositoryInvoker.invokeFindOne(UnwrappingRepositoryInvokerFactory.java:130) ~[spring-data-rest-core-2.6.4.RELEASE.jar:na]
    at org.springframework.data.rest.webmvc.RepositoryEntityController.getItemResource(RepositoryEntityController.java:524) ~[spring-data-rest-webmvc-2.6.4.RELEASE.jar:na]
    at org.springframework.data.rest.webmvc.RepositoryEntityController.getItemResource(RepositoryEntityController.java:335) ~[spring-data-rest-webmvc-2.6.4.RELEASE.jar:na]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_111]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_111]
    ...

A separate custom EntityLookupSupport<MyModel> component class works.

Am I missing something to get it working for Long using method references in my RepositoryRestConfigurerAdapter?