Why still get into srcache_store handler even if $srcache_expire=0?

I just put my srcache settings directly under server block:

server {
    set $canonical_host www.example.com;

    # ...

    srcache_default_expire 0;
    set_escape_uri $pagecache_key $canonical_host$uri$is_args$args;
    srcache_fetch GET /pagecache key=$pagecache_key;
    srcache_store PUT /pagecache key=$pagecache_key&exptime=$srcache_expire;
    include srcache_status_headers;

    # ...
}

You see, I use srcache_default_expire, So the $srcache_expire will default to 0,
I expect my upstream setting their appropriate exptime by max-age.

However, my srcache_store get involved even if $srcache_expire==0,
and then cause an extra useless redis request.

location = /redis_put {
    # tested OK with SSDB
    internal;
    set_unescape_uri $exptime $arg_exptime;

    redis2_query setx $redis_key $echo_request_body $exptime;

    redis2_pass pagecache_ssdb;
}

Start by tracing the srcache_default_expire and srcache_store directive handling in the module and reproduce the supplied server configuration. Check whether an expire value of 0 is intended to suppress the store subrequest; done means avoiding the extra Redis request while preserving upstream max-age behavior.