Cumber: Sending unwanted form parameters


I have a 2 model form. Vehicle which accepts_nested_attributes_for

The form has a single File input field for an image. The form does not
require a file to process (Works in Dev fine).

In cucumber I fill out some fields then submit. I'm getting back
Validation errors from the Image model. "Data file name must be set."
But I'm not submitting an image so the Image model should never be
called. Why is cucumber sending data that's getting to the nested
attributes in vehicle?