ClassNotFoundException when using ActionBarSherlock starting Fragment with "newInstance" - tabs

Question

I am converting Android 4.x code to use ActionBarSherlock so that our App can be compatible with Gingerbread.
So far so good, but it fails launching a new instance of a fragment.
My MainActivity extends SherlockFragmentActivity implements ActionBar.TabListener.
The code fails here where case is 0:
#Override
public Fragment getItem(int position) {
switch (position) {
case 0:
mFragmentProjects = ProjectsFragment.newInstance(position);
return mFragmentProjects;
case 1:
mFragmentContacts = FragmentPeople.newInstance(position, 0);
return mFragmentContacts;
}
return ArrayListFragment.newInstance(position);
}
Where case is 0 it supposed to initialize the fragment but I get this exception:
ClassNotFoundException. The only other clue I have is:
"this" in PathClassLoader and in "name" it says android.app.ActionBar$TabListener
I guess this has something to do with TabListener or libraries not included / loading correctly? I have already cleaned the project.
The fragment ProjectsFragment extends SherlockListFragment.
newInstance is pretty straitforward:
static ProjectsFragment newInstance(int num) {
ProjectsFragment f = new ProjectsFragment();
// Supply num input as an argument.
Bundle args = new Bundle();
args.putInt("num", num);
f.setArguments(args);
return f;
}

Answer 1

It turns out that although MainActivity has no reference to ActivityY, the mere fact that ActivityY did not have the SherlockFragment code made it fail. To describe this differently:
MainActitivy uses ProjectsFragment which is a list
When you click on a list in ProjectsFragment it calls ListsActivity
ListsActivity references ItemsFragment
I had to change ItemsFragment to Sherlock code before MainActivity would work. It seems Java "looks ahead" in some way when you're working with pagers and tabs and fragments.