vertx error with JsonObject

@Grapes([
    @Grab(group='io.vertx', module='vertx-core', version='3.4.1'),    
    @Grab(group='io.vertx', module='vertx-lang-groovy', version='3.4.1')
])
import io.vertx.core.*
import io.vertx.core.json.*

JsonObject jo1 = new JsonObject()

assert "jo1 is a ${jo1?.class?.name}" == "jo1 is a io.vertx.core.json.JsonObject"

assert (jo1 as Map) instanceof Map


JsonObject jo2 = new JsonObject([:])

assert "jo2 is a ${jo2?.class?.name}" == "jo2 is a io.vertx.core.json.JsonObject"

assert (jo2 as Map) instanceof Map


JsonObject jo3 = new JsonObject([a:1])

assert "jo3 is a ${jo2?.class?.name}" == "jo3 is a io.vertx.core.json.JsonObject"

try {
    println (jo3 as Map)
} catch (Exception e) {
    assert true:"An exception has been thrown"
}