Unable to select view fields for use with custom action

Hi,

Here is a view I've created, with a custom action I've just added. I've had to add the custom action using the view definition free-script editor, because I was unable to select the field "FullEpisodeNumber" as argument for the function in the "create custom action" wizard. Why??

list Subs_a_bajar
{
    displayname = "Subs a bajar"
    show all rows from Capitulos [Sub == false]
    (
        Serie
        FullEpisodeNumber
        Title
        Filename
        DateAdded
    )
    filters
    (
        Serie
        "Ready to watch" : (Sub && Seen == false)
        "Need subs" : Sub == false
    )
    group by
    (
        Serie   ascending
    )
    options
    (
        display rows = 30
    )
    custom actions
    (
        "Subs bajados"
        (
            function = SubBajado(Serie, FullEpisodeNumber)
            show action in view header = true
            show action for each record = false
        )
    )
}

here is the code of SubBajado() function:

void SubBajado(string S, string E)
{
    // Recuperamos el capitulo visto (x)
    info "S=" + input.S;
    info "E=" + input.E;
    x = Capitulos [(Serie == input.S && FullEpisodeNumber == input.E)];
    x.Sub = true;
    thisapp.SetFirstNonSeen(input.S);
}

When I select a row in the view, and run the custom action for it, E (FullEpisodeNumber passed from the view to the function) appears to be null inside the function (while actually FullEpisodeNumber isn't null in the view)!

I get S=some value
E=null
and then the funcion fails (because E couldn't be null).

Some help? Is it a bug?
Regards,
Alex