Mocking a Singleton class for junit testing

I am new to JUnit testing. I want to mock a singleton class. The class looks like this.

public enum MockClass{
    INSTANCE;

    private MockClass(){
         //private constructor
    }

    public void convertToString(int a){
         return SomeClass.setString(a.toString());
    }

}

I want to mock this class such that whenever there is a call to MockClass.INSTANCE.convertToString(a), nothing should be executed. There is a function in PowerMockito called doNothing(). But what do I do for Singleton classes as shown above? In this case PowerMockito doesn't help.