How to use $request_body?

[witekfl Gazeta.pl]

Hi,
There is in config:
location / {
    root blabla;
    if ($request_body ~* (blabla|bebe)) {
         return 404;
    }
}
nginx -t says invalid condition "$request_body".
Tell me what I'm doing wrong.

If you don't want to answer, tell me why and I'll stop asking.
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