background scroll on modal form

hi

if you go here

http://www.angelapersonaltuscantour.com/package/chianti-and-castle-wine-tour/

and you click on “book now” link, a form modal is opened. But when i try to scroll this form, the background scrolls, not the form!!!

how can i fix it?
thanks

bye

https://ww.wp.xz.cn/plugins/ninja-forms-modal/