@Test(expected=IllegalArgumentException.class)
public void testNull() throws Exception {

    MolecularFormulaRange mfRange = new MolecularFormulaRange();
    IIsotope carb = builder.newInstance(IIsotope.class, "C");
    IIsotope nul = null;
    mfRange.addIsotope(carb, 2, 5);
    mfRange.addIsotope(nul, 3, 7);
}