¿Cómo establecer el margen inferior a FAB en el layout Anko DSL?

Uso el layout DSL de Anko en mi actividad de kotlin. No puedo establecer el margen inferior a FAB. Trabajo de margen derecho.

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En mi actividad

override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) (application as SamfantozziApp).dgaeacomponent().inject(this) InvoiceListKtActivityUI(_rxBus).setContentView(this) } 

Mi layout Anko DSL InvoiceListKtActivityUI.kt

 class InvoiceListKtActivityUI (val _rxBus: RxBus): AnkoComponent<InvoiceListKtActivity>{ override fun createView(ui: AnkoContext<InvoiceListKtActivity>): View = with(ui){ return relativeLayout{ padding = dip(5) lparams { width = matchParent height = wrapContent margin = 5 } verticalLayout{ tabLayout{ lparams { width = matchParent height = wrapContent } id = R.id.tabs } viewPager{ lparams { width = matchParent height = matchParent } id = R.id.container } } floatingActionButton{ lparams { width = wrapContent height = wrapContent rightMargin = 40 //works bottomMargin = 40 //does not work alignParentBottom() alignParentRight() } imageResource = android.R.drawable.ic_input_add id = R.id.fabinvoice onClick{ _rxBus.send(InvoiceListFragment.ClickFobEvent()) } } } } }