Test for parameterisation of a class

In my test I have this:

given:
def dueDateCol = App.instance.fxmlController.treeTableView.columns.get( 1 )

when:
def editorCell = dueDateCol.cellFactory.call( dueDateCol )

then:
editorCell instanceof DueDateEditor<TaskItem, LocalDate>

My app code is currently this:

class DueDateEditor extends TreeTableCell {
}

... but the test passes! I want it to fail until the app code is correctly parameterised:

class DueDateEditor extends TreeTableCell<TaskItem, LocalDate>{
}