Problem with listbox

[#1]

Hello everybody, i have created a list box, i have used a field with an expression, but when i select an item, the expression becom null. See the picture. How can i solve the problem?

[Gysbert_Wassenaar]

Try =Only({1} ANO_TIPO_PORTO_descr)

[#3]

Yes!!! Works, thank you so much!!!