Strange behaviour with Swingbuilder.list and listData casting to Vector

import groovy.swing.*
import javax.swing.*

def swing = new SwingBuilder()

def masterData = [en: ['Hello, World!', 'How are you?'], 
                  es: ['Hola Mundo!', "Como Esta?"], 
                  can: ['Foon Ying!', 'Lei Ho Ma?']]

def detailModel = new DefaultListModel()

swing.actions() {
  action(id: 'masterAction',
          name: 'masterAction',
          closure: {e->
            def l = e.source
            if (!e.valueIsAdjusting) {
              def i = l.selectedValue
              detailModel.clear()
              masterData[i].each {
                detailModel.addElement(it)
              }
            }
          })
}
swing.frame(title: 'My Frame', pack: true, visible: true,
                 defaultCloseOperation: javax.swing.WindowConstants.EXIT_ON_CLOSE) 
{
  panel() {
    tableLayout() {
      tr {
        td {
          scrollPane() {
            list(valueChanged: masterAction.closure, 
                 visibleRowCount: 3, 
                 listData: masterData.collect {k, v -> k })
          }
        }
        td {
          scrollPane() {
            list(visibleRowCount: 3, model: detailModel)
          }
        }
      }
    }
  }
}

Exception thrown: Cannot cast object '[en, es, can]' 
with class 'java.util.ArrayList' to class 'java.util.Vector'

org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object 
'[en, es, can]' with class 'java.util.ArrayList' to class 'java.util.Vector'
	at ConsoleScript0$_run_closure2_closure5_closure6_closure7_closure8_closure10.
            doCall(ConsoleScript0:31)
	at ConsoleScript0$_run_closure2_closure5_closure6_closure7_closure8_closure10.
            doCall(ConsoleScript0)
	at ConsoleScript0$_run_closure2_closure5_closure6_closure7_closure8.
            doCall(ConsoleScript0:30)
	at ConsoleScript0$_run_closure2_closure5_closure6_closure7_closure8.
            doCall(ConsoleScript0)
	at ConsoleScript0$_run_closure2_closure5_closure6_closure7.
            doCall(ConsoleScript0:29)
	at ConsoleScript0$_run_closure2_closure5_closure6_closure7.
            doCall(ConsoleScript0)
	at ConsoleScript0$_run_closure2_closure5_closure6.doCall(ConsoleScript0:28)
	at ConsoleScript0$_run_closure2_closure5_closure6.doCall(ConsoleScript0)
	at ConsoleScript0$_run_closure2_closure5.doCall(ConsoleScript0:27)
	at ConsoleScript0$_run_closure2_closure5.doCall(ConsoleScript0)
	at ConsoleScript0$_run_closure2.doCall(ConsoleScript0:26)
	at ConsoleScript0$_run_closure2.doCall(ConsoleScript0)
	at ConsoleScript0.run(ConsoleScript0:24)

list(..., listData: masterData.collect {k, v -> k } as Object[])

"C:\Program Files\Java\jdk1.6.0_16\bin\java" -Dgroovy.home=C:\DEVTOOLS\groovy-1.6.4
 -Dgroovy.starter.conf=C:\DEVTOOLS\groovy-1.6.4\conf\groovy-starter.conf 
"-Dtools.jar=C:\Program Files\Java\jdk1.6.0_16\lib\tools.jar" -Didea.launcher.port=7532 
"-Didea.launcher.bin.path=C:\Program Files (x86)\JetBrains\IntelliJ IDEA 10666\bin" 
-Dfile.encoding=windows-1252 -classpath "C:\DEVTOOLS\groovy-1.6.4\lib\groovy-1.6.4.jar;
C:\Program Files (x86)\JetBrains\IntelliJ IDEA 10666\lib\idea_rt.jar" 
com.intellij.rt.execution.application.AppMain org.codehaus.groovy.tools.GroovyStarter 
--conf C:\DEVTOOLS\groovy-1.6.4\conf\groovy-starter.conf --classpath 
C:\DEVELOPMENT\CascadingLists\out\production\CascadingLists 
--main groovy.ui.GroovyMain 
C:\DEVELOPMENT\CascadingLists\src\CascadingLists.groovy

C:\Windows\System32>java -version
java version "1.6.0_16"
Java(TM) SE Runtime Environment (build 1.6.0_16-b01)
Java HotSpot(TM) 64-Bit Server VM (build 14.2-b01, mixed mode)

C:\DEVTOOLS\groovy-1.7-beta-1\bin>set CLASSPATH=
C:\DEVTOOLS\groovy-1.7-beta-1\bin>set GROOVY_HOME=C:\DEVTOOLS\groovy-1.7-beta-1
C:\DEVTOOLS\groovy-1.7-beta-1\bin>groovyconsole.bat