Mockito -从服务调用方法时的NullpointerException

public interface SomeService {
    String someMethod(String param);
}

public class SomeClass {
    private SomeService someService;

    public SomeClass(SomeService someService) {
        this.someService = someService;
    }

    public String someMethod() {
        return someService.someMethod("testParam");
    }
}

import org.junit.Before;
import org.junit.Test;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.MockitoAnnotations;

import static org.mockito.Mockito.when;
import static org.junit.Assert.assertEquals;

public class SomeClassTest {

    @Mock
    private SomeService someService;

    @InjectMocks
    private SomeClass someClass;

    @Before
    public void setUp() {
        MockitoAnnotations.initMocks(this);
    }

    @Test
    public void testSomeMethod() {
        when(someService.someMethod("testParam")).thenReturn("expectedValue");

        String result = someClass.someMethod();
        assertEquals("expectedValue", result);
    }
}