טו כְּדָת מַה–לַּעַשׂוֹת בַּמַּלְכָּה וַשְׁתִּי עַל ׀ אֲשֶׁר לֹא–עָשְׂתָה אֶת–מַאֲמַר הַמֶּלֶךְ אֲחַשְׁוֵרוֹשׁ בְּיַד הַסָּרִיסִים
15. The rule: what is done to the Queen Vashti regarding her not performing the message of King Achashverosh through the chamberlains?
Truly, being king, Achashverosh was neither bound by the law nor ignorant of it. Therefore, according to Midrash Panim Acheirim, the rule to which Achashverosh refers would be a legal loophole to try to save Vashti.