Mock final class: powermock + easymock + testng

I am new to PowerMock and cannot find an example that fits my situation.

This is nothing complicated. I am trying to use PowerMock + EasyMock to mock the last class in TestNG.

Here is the last class:

public final class MyFinalClass {
    private String myString = "someString";

    public final String getMyString() {
        return myString;
    }
}

The tested class passes an instance of the final class:

public class MyActiveClass {
    private MyFinalClass finalClass;

    public String getFinalString() {
        return finalClass.getMyString();
    }

    public void setFinalClass(MyFinalClass finalClass) {
        this.finalClass = finalClass;
    }
}

And here is the Test class:

import static org.easymock.EasyMock.expect;
import org.powermock.api.easymock.PowerMock;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.testng.PowerMockTestCase;
import org.testng.Assert;
import org.testng.annotations.Test;


@PrepareForTest({ MyFinalClass.class })
public class MyTestClass extends PowerMockTestCase {

    private MyActiveClass activeClass = new MyActiveClass();
    private MyFinalClass mockFinalClass;

    @Test
    public void getFinalString() {
        String expectedString = "mockedReturnString";
        mockFinalClass = PowerMock.createMock(MyFinalClass.class);
        activeClass.setFinalClass(mockFinalClass);
        expect(mockFinalClass.getMyString()).andReturn(expectedString);
        PowerMock.replayAll();
        Assert.assertEquals(activeClass.getFinalString(), expectedString);
        PowerMock.verifyAll();
    }
}

But when I run the test, I get an invalid argument exception complaining that MyFinalClass is not an interface:

FAILED: getFinalString
java.lang.IllegalArgumentException: MyFinalClass is not an interface
    at java.lang.reflect.Proxy.getProxyClass(Unknown Source)
    at java.lang.reflect.Proxy.newProxyInstance(Unknown Source)
    at org.easymock.internal.JavaProxyFactory.createProxy(JavaProxyFactory.java:24)
    at org.easymock.internal.MocksControl.createMock(MocksControl.java:51)
    at org.powermock.api.easymock.PowerMock.doCreateMock(PowerMock.java:2212)
    at org.powermock.api.easymock.PowerMock.doMock(PowerMock.java:2163)
    at org.powermock.api.easymock.PowerMock.createMock(PowerMock.java:89)
    at MyTestClass.getFinalString(MyTestClass.java:22)
         ...

What am I missing? Thanks in advance for your help!