Problem with $request_body

[BruXy]

I think, that this example from documentation echo_read_request_body of does not work:

location / {
     echo_read_request_body;
     echo "Start";
     echo $request_body; # <- NOTHING
     echo $echo_request_body; # <- data OK
     echo "Stop";
}

I was digging for $request_body and it has no value, data provided in POST request are in $echo_request_body.

What should be the correct behavior?