Mockito: Trying to spy on method is calling the original method

I’m using Mockito 1.9.0. I want mock the behaviour for a single method of a class in a JUnit test, so I have

final MyClass myClassSpy = Mockito.spy(myInstance);
Mockito.when(myClassSpy.method1()).thenReturn(myResults);

The problem is, in the second line, myClassSpy.method1() is actually getting called, resulting in an exception. The only reason I’m using mocks is so that later, whenever myClassSpy.method1() is called, the real method won’t be called and the myResults object will be returned.

MyClass is an interface and myInstance is an implementation of that, if that matters.

What do I need to do to correct this spying behaviour?