Question about setup error in mac80211

[Larry Finger <Larry.Finger@lwfinger.net>]

In https://bugzilla.kernel.org/show_bug.cgi?id=43044, driver rtl8192ce is 
getting an oops due to a NULL pointer dereference due to NULL being returned by 
ieee80211_get_tx_rate. For reference, the code in question is

static inline struct ieee80211_rate *
ieee80211_get_tx_rate(const struct ieee80211_hw *hw,
                       const struct ieee80211_tx_info *c)
{
         if (WARN_ON(c->control.rates[0].idx < 0))
                 return NULL;
         return &hw->wiphy->bands[c->band]->bitrates[c->control.rates[0].idx];
}

Any thoughts regarding what might be causing this? I don't have the full log, 
but I assume that the WARN_ON must have been in the logs.

I can easily protect rtlwifi, but I would like to know the cause.

Thanks,

Larry