How to mock An Interface Java PowerMockito

I m trying to mock an interface.

public interface FlowCopyParamsBusinessManager {
List<FlowCopyParams> findByAppli(String application, String sourcePattern)
        throws FlowCopyParamsBusinessException;

}

In my code, when i call this method findByAppli, i would like to return a list of FlowCopyParams.

List<FlowCopyParams> lstFlowCopyParams = flowCopyParamsBusinessManager.findByAppli(
                    "TOTO","TATA);

Here my try in the class test:

@BeforeClass
public static void mockBeanIn() throws Exception {
List<FlowCopyParams> flowCopyParamsList = new ArrayList<>();

PowerMockito.spy(FlowCopyParamsBusinessManager.class);
PowerMockito.when(FlowCopyParamsBusinessManager.class, "findByAppli",  Mockito.anyString(), Mockito.anyString()).thenReturn(flowCopyParamsList);
}  

I have this error :

java.lang.IllegalArgumentException: object is not an instance of declaring class

I don't know why because the method findByAppli must have two string parameters, and i put Mockito.anyString() and i still have IllegalArgumentException.

Any clue ?

Thxs.