Convert to Brazilian Cities Class

Question

Gentlemen, I have two combos in one form, the first of which is loaded with the Brazilian states, via routine database. It is passed the acronym of the state chosen for a routine in my control, which calls a routine in my persistence class, with the parameter. The routine works (I see it in step-by-step), but when loading the combo, it generates the following exception: "Validation error: the value is necessary". I don’t know how to make the class convert. Below follows excerpt from the code: