Grails - JSONBuilder - Spec toPrettyString() returns stackoverflow

I m making a unit test that needs to return a Json. To build that json i m using toPrettyString() method from JSONBuilder.

This is the class to spec:

class Lugar {
String sigla
String nombre
Coordenada coordenada

String toString(){
    "${sigla}"
}

String toJson()
{
    new JsonBuilder( this ).toPrettyString()
}

static constraints = {
    nombre blank: false , nullable: false
}
}

The spec to run is this:

@TestFor(Lugar)
class LugarSpec extends Specification {

void "toJson not empty"() {

    when:
    Lugar lugar = new Lugar(sigla: "BUE", nombre:"BUENOS AIRES")
    String aux = lugar.toJson();

    then:       
    ! aux.dump().empty

}
}

But the result is:

 <error type="java.lang.StackOverflowError">java.lang.StackOverflowError
    at org.springsource.loaded.ri.ReflectiveInterceptor.jlrMethodInvoke(ReflectiveInterceptor.java:1270)
    at groovy.lang.MetaBeanProperty.getProperty(MetaBeanProperty.java:60)
    at groovy.lang.PropertyValue.getValue(PropertyValue.java:40)
    at groovy.json.JsonOutput.writeObject(JsonOutput.java:287)
    at groovy.json.JsonOutput.writeMap(JsonOutput.java:421)
    at groovy.json.JsonOutput.writeObject(JsonOutput.java:291)
    at groovy.json.JsonOutput.writeArray(JsonOutput.java:326)
    at groovy.json.JsonOutput.writeObject(JsonOutput.java:283)
    at groovy.json.JsonOutput.writeMap(JsonOutput.java:421)
    at groovy.json.JsonOutput.writeObject(JsonOutput.java:291)
    at groovy.json.JsonOutput.writeArray(JsonOutput.java:326)
    at groovy.json.JsonOutput.writeObject(JsonOutput.java:283)
    at groovy.json.JsonOutput.writeMap(JsonOutput.java:421)
    at groovy.json.JsonOutput.writeObject(JsonOutput.java:291)
    at groovy.json.JsonOutput.writeArray(JsonOutput.java:326)
    at groovy.json.JsonOutput.writeObject(JsonOutput.java:283)
    at groovy.json.JsonOutput.writeMap(JsonOutput.java:421)
    at groovy.json.JsonOutput.writeObject(JsonOutput.java:291)
    at groovy.json.JsonOutput.writeArray(JsonOutput.java:326)
    at groovy.json.JsonOutput.writeObject(JsonOutput.java:283)
    at groovy.json.JsonOutput.writeMap(JsonOutput.java:421)

And it continues repeating until the end.

I tried to test toJson on a main and the results were fine:

    static void main(String[] args) {
    Lugar lugar = new Lugar(sigla: "BUE", nombre:"BUENOS AIRES")

    String aux = lugar.toJson();

    println aux.dump()
}

The results were:

{
"sigla": "BUE",
"constraints": {
    "nombre": {
        "blank": false,
        "nullable": false
    }
},
"nombre": "BUENOS AIRES"
}

Well i'd really appreciate i m trying to make some TDD and i m new on Grails.

Thanks.