Parse JSON Object from URL in Groovy

I'm trying to parse the JSON object from the following API into groovy:

http://mtgapi.com/api/v1/fetch/id/1?token=f1fc6636e6f25d97c007984f0c7fe5785b3e3482

Here is my class:

package mtgtournamentorganizer

import groovy.json.JsonSlurper

class GetCardService {

    String token = "?token=f1fc6636e6f25d97c007984f0c7fe5785b3e3482"
    String base = "http://mtgapi.com/api/v1/fetch/"
    String id = "id/"
    String cardId
    String apiString

    def getCardById(cardId) {

        apiString =base + id + cardId + token

        URL apiUrl = new URL(apiString)

        def card = new JsonSlurper().parse(apiUrl)

        return card

    }

}

When I call getCardById(1)

I get this error:

|  groovy.lang.MissingMethodException: No signature of method: groovy.json.JsonSlurper.parse() is applicable for argument types: (java.net.URL) values: [http://mtgapi.com/api/v1/fetch/id/1?token=f1fc6636e6f25d97c007984f0c7fe5785b3e3482]
Possible solutions: parse(java.io.Reader), use([Ljava.lang.Object;), wait(), any(), grep(), wait(long)
    at mtgtournamentorganizer.GetCardService.getCardById(GetCardService.groovy:21)