refreshCheck: '[null_mem_obj]' ?

Squid 1.2.beta22

I've noticed that refreshCheck() sometimes gets "[null_mem_obj]" instead
of a URL:

    refreshCheck: '[null_mem_obj]'
    refreshCheck: Matched '. 0 20% 259200'
    refreshCheck: age = 1833
    refreshCheck: factor = 0.027338
    refreshCheck: NO: factor < pct

This appears to be interfering with some of my refresh_patterns. I've
seen that "[null_mem_obj]" is returned by storeUrl() but I'm not sure why:

    const char *
    storeUrl(const StoreEntry * e)
    {
        if (e == NULL)
            return "[null_entry]";
        else if (e->mem_obj == NULL)
            return "[null_mem_obj]";
        else
            return e->mem_obj->url;
    }

Under what circumstances is e->mem_obj set to NULL?

-- 
Michael Fuhr
http://www.fuhr.net/~mfuhr/