PowerMock(with mockito) simple unit test error

I am using Mockito + PowerMock to write a simple unit test for the following singleton class:

public class MyService {
   private static MyService service;
   private List<School> schoolList;   

   private MyService(){
      // test case error complains here!
      School school = new School(); 
      schoolList.add(school);
   }

   public static Singleton getInstance( ) {
      return service;
   }

   protected static void printSchool( ) {
      School school = schoolList.get(0);
      print(school);
   }
}

My Test case:

@RunWith(PowerMockRunner.class)
public class MyServiceTest {

  @PrepareForTest({MyService.class})
  @Test
  public void testPrintSchool() {
     // enable mock static function
     PowerMockito.mockStatic(MyService.class);

     MyService mockService = PowerMockito.mock(MyService.class);
     PowerMockito.when(MyService.getInstance())
                  .thenReturn(mockService);
  }

}

I run my test, but got the following error:

java.lang.RuntimeException: Invoking the beforeTestMethod method on PowerMock test listener org.powermock.api.extension.listener.AnnotationEnabler@3ab19451 failed.
    at com.xyz.MyService.<init>(MyService.java:12)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:526)
    at org.mockito.internal.util.reflection.FieldInitializer$ParameterizedConstructorInstantiator.instantiate(FieldInitializer.java:257)
    at org.mockito.internal.util.reflection.FieldInitializer.acquireFieldInstance(FieldInitializer.java:124)
    at org.mockito.internal.util.reflection.FieldInitializer.initialize(FieldInitializer.java:86)
    at org.mockito.internal.configuration.injection.ConstructorInjection.processInjection(ConstructorInjection.java:52)
...

As you can see, the error complains about MyService.java:12 line 12, that is the line School school = new School(); in MyService constructor.

Why I get this error, how to get rid of it?