Why doesn't this code work in the source view

Question

I have this piece of code:

                    Reps.Algo _Rep = new Reps.Algo();
                    Model.Entities.G guia = new Model.Entities.G;

                    guia = _Rep.GetById(GuiaId);

                    foreach(var un in guia.Unidades)
                    {
                        Do Something
                    }

When I place it on the Code-Behind it works perfectly, but when I place it in the Source view of the page it says:

"foreach statement cannot operate on variables of type entity collection"

What I'm I missing here???