"object is not an instance of declaring class" thrown invoking a method on Groovy class, wrapped in a Spring proxy

Exception in thread "main" java.lang.IllegalArgumentException: object is not an instance of declaring class
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
        at java.lang.reflect.Method.invoke(Unknown Source)
        at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:86)
        at groovy.lang.MetaMethod.doMethodInvoke(MetaMethod.java:226)
        at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:899)
        at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:740)
        at org.codehaus.groovy.runtime.InvokerHelper.invokePogoMethod(InvokerHelper.java:777)
        at org.codehaus.groovy.runtime.InvokerHelper.invokeMethod(InvokerHelper.java:757)
        at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.invokeMethodN(ScriptBytecodeAdapter.java:167)
        at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.invokeMethod0(ScriptBytecodeAdapter.java:195)
        at X.doIt(script1.groovy]:5)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
        at java.lang.reflect.Method.invoke(Unknown Source)
        at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:301)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:182)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:149)
        at org.springframework.aop.support.DelegatingIntroductionInterceptor.doProceed(DelegatingIntroductionInterceptor.java:131)
        at org.springframework.aop.support.DelegatingIntroductionInterceptor.invoke(DelegatingIntroductionInterceptor.java:119)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171)
        at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:204)
        at $Proxy1.doIt(Unknown Source)
        at TestMe.main(TestMe.java:20)

class X implements Doable {
    def prop
    void doIt() {
        println prop.getClass().getName();
        prop.doIt(); 
    }
}

class Y {
    void doIt() { println "OK"; }
}

public interface Doable {
    void doIt();
}

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:lang="http://www.springframework.org/schema/lang"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
       http://www.springframework.org/schema/lang http://www.springframework.org/schema/lang/spring-lang-2.5.xsd">
    
    <lang:groovy id="y" script-source="classpath:groovy/script2.groovy" refresh-check-delay="1000"/>
    <lang:groovy id="x" script-source="classpath:groovy/script1.groovy" refresh-check-delay="1000">
        <lang:property name="prop" ref="y"/>
    </lang:groovy>
</beans>

import org.springframework.context.support.ClassPathXmlApplicationContext;
public class TestMe {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext ctxt = new ClassPathXmlApplicationContext("spring.xml");
        Doable o = (Doable)ctxt.getBean("x");
        o.doIt();
    }
}