Mapping non-empty $query_string (no replies)

Hi!
I'm running a Wordpress based website on my server, I've set up fastcgi cache recently. Now I want to get rid of evil-ifs, that are used to detect cache skipping cases, and replace them with map.

I managed to convert all common ifs except this one,

if ($query_string != "") {
set $skip_cache 1;
}

I'm trying to replace it with non-evil map in http section.

#skip cache if query string has at least one character
map $query_string $skip_cache {
"~.+" "1";
}

For some reason it doesn't work as intended. It starts bypassing cache on ALL pages except main page.

Thanks for any help.
Alex