I have docs in my index, where every doc has a field popularity, which is a float.

What I want: multiply the field popularity by the score value (computed using bf) and order the docs by popularity * score desc.

what I tried: "sort":"mul(score, popularity) desc", which gives me the error "Can't determine a Sort Order (asc or desc) in sort spec 'mul(score, popularity_v2.fr) desc', pos=10"

Thanks for your help!