are humble store gift urls supported as "keys?"

i'm trying to make a giveaway for two copies of a game i bought through the humble store. i put the two urls in the keys box like this (gift ids removed of course, but are 16 characters):

https://www.humblebundle.com/s?gift=SECRETGIFTIDHERE
https://www.humblebundle.com/s?gift=SECRETGIFTIDHERE

it says 2 keys detected, but when i try to preview my giveaway it says "Only Steam redeemable keys, or gift links from Steam or Humble Bundle are allowed." i sort of assumed humble store gift links look the same as humble bundle gift links. am i wrong? do i need to set this up as a 2-copy gift giveaway and send out the gift urls manually?