Jade Mixin: 500 TypeError: Cannot read property 'length' of undefined

I've got a mixin causing me trouble:

mixin dialog(title, description, choices)
    form.choices.dialog.row
        legend
            h1 #{title}
            p.description #{description}
        fieldset
            for choice in choices
                label
                    input(type= "radio", name = "choice.name", checked = "checked", required = "required")
                    | choice.name
        div.row.form-actions
            button(type="submit", ) Make Choice

To call it, I first include the mixin file with this mixin:

import dialog

Then I use the mixin, after creating a javascript variable:

- var investiageUFODialog = [{headerText: "Would you like to investiage the UFO?", description: "Choose whether or not to investigate the UFO."}, {choices: [{name: "Yes"}, {name: "No"}]}]

mixin dialog(investiageUFODialog.headerText, investiageUFODialog.description, investiageUFODialog.choices)

What am I doing wrong?