Method Reference throwing null pointer

[asheshsaraf]

public static <T, Z> T getIfNotNull(final Z object, final Supplier objectValue) {
return object == null ? null : objectValue.get();
}

@test
public void test() {
final String id = null;
MatcherAssert.assertThat(ConditionalOperation.getIfNotNull(id, () -> id.toString()) == null, Is.is(true)); // working
MatcherAssert.assertThat(ConditionalOperation.getIfNotNull(id, id::toString) == null, Is.is(true));
// not working
}