为什么在尝试从mock返回值时会出现编译时错误？

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 SomeServiceTest {

    @Mock
    private SomeClass someClass;

    @InjectMocks
    private SomeService someService;

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

    @Test
    public void testSomeMethod() {
        when(someClass.someMethod()).thenReturn("expectedValue");
        String result = someService.someMethod();
        assertEquals("expectedValue", result);
    }
}