They don't check for them, they just forcibly convert them into ASCII by taking the lowest byte of the Unicode code point, turning "ⅠⅡⅢⅣⅤⅥⅦⅧⅨⅩⅪⅫⅬⅭⅮⅯⅰⅱⅲⅳⅴⅵⅶⅷⅸⅹⅺⅻⅼⅽⅾⅿ" into "`abcdefghijklmnopqrstuvwxyz{|}~\x7f". A few of these characters get rejected, though curiously not the last one, which is nonprintable.