error in instance initializer for Enum element, without special method

enum Color {
    RED {
      { println("foo"); } // Instance initalizer
    },GREEN,BLUE
}

$ groovy  enumtest.groovy
org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
/Users/uehaj/src/groovy/enumtest.groovy: -1: You are not allowed to overwrite the final method $INIT([Ljava.lang.Object;) from class 'Color'.
 @ line -1, column -1.
1 error

num Color {
    RED {
      String toString() {}
      { println("foo"); } // Instance initalizer
    },GREEN,BLUE
}