"object Object" response in Google Floodlight custom variable

Question

I am utilizing the Google Tag Manager javascript variable to pass through a custom array into a Floodlight pixel u-variable. Within GTM preview, the array is populating correctly. However, my floodlight custom variable reporting returns a [object Object] value and not the array.

Two questions:

1. What is the reason for the "object Object" value response?
2. Is passing an array into a custom variable not allowable?

Thanks in advance to the community for the help.

GTM Variable: order_item_data.items

Here is the GTM response. My variable is populating correctly.