iftab_helper return an already allocated devname on ppp with /etc/iftab

When I used ppp with ordinary /etc/iftab, iftab_helper failed to prove devicename.

test call:
># /lib/udev/iftab_helper ppp0
>eth1

in syslog:
>Dec 2 05:23:42 undine iftab_helper[27857]: selector_value: unable to access parent device of '/class/net/ppp0'
>Dec 2 05:23:42 undine iftab_helper[27857]: whats_my_name: /etc/iftab:1: unknown or unsupported mac selector for eth0
>Dec 2 05:23:42 undine iftab_helper[27857]: selector_value: unable to access parent device of '/class/net/ppp0'
>Dec 2 05:23:42 undine iftab_helper[27857]: whats_my_name: /etc/iftab:2: unknown or unsupported mac selector for eth1

/etc/iftab:
>eth0 mac 00:13:72:1a:10:7b
>eth1 mac 00:0e:0c:82:cc:07