How can i fix this error? c# asp linq

A few minutes ago this part of my code was functional with success, few minutes after i see this when i compile the project

And the code is:

    string Cat = ddlCategoria.SelectedValue;
    int? MaxIdCat = (from p in db.tblProducto
                     where p.IdCategoria == Cat
                     select p.IdProducto).Max() + 1;

    txtIdEmp.Text = Convert.ToString(MaxIdCat ?? +1);