progress percentage is NaN when on promo.html

example page: https://books-are-next.github.io/krest-svateho-vladimira/promo.html
how it looks:
<img width="124" alt="Screenshot 2022-06-30 at 22 19 36" src="https://user-images.githubusercontent.com/24832873/176771432-7cfce23b-bdde-42c7-bfd4-adc9160182ea.png">