Partial Mock Private Method with Powermock-Mockito

I am new to Mockito and PowerMock. I need to test some legacy code which has a private method I have to mock. I am considering using the private partial mocking feature from PowerMock, I tried to mimic the example from the link, but it failed. I have no idea what's wrong with it. Could you help to check it? Thanks

Here's the class to-be-tested:

package test;

public class ClassWithPrivate
{

     private String getPrivateString() {
       return "PrivateString";
     }

     private String getPrivateStringWithArg(String s) {
       return "PrivateStringWithArg";
     }

 }

And This is the Test Code:

package test;

import static org.mockito.Mockito.*;
import static org.mockito.Matchers.anyString;
import static org.powermock.api.mockito.PowerMockito.when;
import static org.powermock.api.support.membermodification.MemberMatcher.method;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.invocation.InvocationOnMock;
import org.mockito.stubbing.Answer;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.api.support.membermodification.MemberMatcher;
import org.powermock.modules.junit4.PowerMockRunner;

@RunWith(PowerMockRunner.class)
@PrepareForTest(ClassWithPrivate.class)
public class ClassWithPrivateTest {

    @Test
    public void testGetPrivateString() {

         ClassWithPrivate spy = PowerMockito.spy(new ClassWithPrivate());

         PowerMockito.doReturn("Do").when(spy, method(ClassWithPrivate.class, "getPrivateStringWithArg", String.class)).withArguments(anyString());

    }

}

EDIT When I tried to compile the code, it failed with the following errors:

ClassWithPrivateTest.java:26: unreported exception java.lang.Exception; must be caught or declared to be thrown
     PowerMockito.doReturn("Do").when(spy, method(ClassWithPrivate.class, "getPrivateStringWithArg", String.class)).withArguments(anyString());
                                     ^
ClassWithPrivateTest.java:26: unreported exception java.lang.Exception; must be caught or declared to be thrown
     PowerMockito.doReturn("Do").when(spy, method(ClassWithPrivate.class, "getPrivateStringWithArg", String.class)).withArguments(anyString());