Grails 2.2.2 UrlMappings testing with exceptions

Is it possible to implement unit test against such url mapping?

"500" (controller: 'error', action: 'handle', exception: MyCustomException)

I've tried to write such unit test

@TestFor(UrlMappings)
@Mock(ErrorController)
class UrlMappingsTest {

    void test() {
        assertForwardUrlMapping(500, controller: "error", action: "handle", exception: MyCustomException)


    }

}

but got junit.framework.AssertionFailedError: url '500' did not match any mappings

If I remove exception: MyCustomException from UrlMapping.groovy and from unit test it works. But I cannot do it.