groovy - この Groovy 式の何が問題になっていますか?

Question

GroovyShell(インタープリター)を介して次の Groovy 式を実行します。

if(fizz.subtype == null) {
    if(fizz.color == 'RED') fizz.subtype = "DOG";
    else if(fizz.color == 'BLUE') fizz.subtype = "CAT";
    else if(fizz.color == 'GREEN') fizz.subtype = "SHEEP";
    else if(fizz.color == 'ORANGE') fizz.subtype = "LION";
    else if(fizz.color == 'YELLOW') fizz.subtype = "SNAIL";
    else if(fizz.color == 'GRAY') fizz.subtype = "SHARK";
    else if(fizz.color == 'PURPLE') fizz.subtype = "BAT";
    else if(fizz.color == 'BLACK') fizz.subtype = "FOX";
}; fizz;

次のスタック トレースが表示されます。

groovy.lang.MissingPropertyException: No such property: subtype for class: com.me.myapp.Fizz
    at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.unwrap(ScriptBytecodeAdapter.java:50)
    at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.getProperty(ScriptBytecodeAdapter.java:479)
    at Script1.run(Script1.groovy:1)
    at groovy.lang.GroovyShell.evaluate(GroovyShell.java:543)
    at groovy.lang.GroovyShell.evaluate(GroovyShell.java:518)
    at com.tms.evaluator.GroovyEvaluator._evaluate(GroovyEvaluator.java:51)
    ...rest of stacktrace omitted for brevity

何か案は？前もって感謝します！