Using jUnit mock objects with Spring

I'm trying to write a unit test for a class that uses Spring. The code itself seems to be fine, but I keep getting a null pointer exception on my When statements for some reason. The source code is as follows:

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = {"/spring-bean-config.xml"}
public class testClass {
    @Mock TestPerson mockTestPerson;
    private TestObject testObject;

    @Before
    public void setup() { testObject = new TestObject(); }

    @Test
    public void testGetFullName() throws Exception {
        String firstname = "Bob";
        String lastname = "Barker";

        when(testPerson.getFirstName()).thenReturn(firstName); // Throws NullPointerException
        when(testPerson.getLastName()).thenReturn(lastName); // I suspect this guy will do the same.

        String result = testObject.getFullName(mockTestPerson);

        assertNotNull(result);
    }
}

The TestPerson class is pretty simple:

public class testPerson {

    private String firstName;
    private String lastName;

    public String getFirstName() {
        return this.firstName;
    }

    public void setFirstName(String lastName) {
        this.lastName = lastName;
    }

    public String getLastName {
        return this.LastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }
}

And finally, the TestObject class.

public class TestObject {
    public String getFullName(TestPerson testPerson) {
        return testPerson.getFirstName + " " + testPerson.getLastName();
    }
}

Simple, yes? To be honest, it might be easier to just create a TestPerson object from scratch. For argument's sake (and for the fact that my other projects that NEED to use @Mock tend to make the same complaints), I need to know how to properly mock object using the @Mock annotation AND SpringJUnit4ClassRunner.