Column with render_* method in django-tables2 not working

I have a table defined like this using django-tables2:

class MyTable(tables.Table):
    action = tables.Column()

    class Meta:
        model = User
        fields = ['name', 'email']

    def render_action(self, record):
        return 'Foo'

But the render_action method is ignored and a -- is printed for every row instead. What am I missing?

I finally solved it adding empty_values=() to the action column attributes.