Powermockito calling actual method when spying

I'm trying to mock a private method using PoweMockito. My actual code is running when I'm trying to tell PowerMockito to spy a method. From the below code, you can see the run method which calls initializeMongo is commented. But still initializeMongo is getting called when I run this test.

package mycompany.project.reader;

import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

@RunWith(PowerMockRunner.class)
@PrepareForTest({ ReaderMainTest.class })
public class ReaderMainTest {

    public static String configFile = "conf/projectreader.test.xml";
    private ReaderMain readerMain = null;

    @Before
    public void setUp() throws Exception {
        readerMain = new ReaderMain(configFile);
    }

    @Test
    public final void testRun() throws Exception {
        ReaderMain readerSpy = PowerMockito.spy(readerMain);
        PowerMockito
                .doNothing()
                .when(readerSpy,
                        PowerMockito
                                .method(ReaderMain.class, "initializeMongo"))
                .withNoArguments();
        // readerSpy.run();
    }
}

Below is the constructor for ReaderMain. So it should not call the run method yet.

public ReaderMain(String configFile) {
        this.configFile = configFile;
}

initializeMongo is a private method in ReaderMain with no arguments and returns void.