Rerolling champion shards

I have three champion shards in my loot. One nunu and two Zac. But when I try to reroll them into a new champion, there's no way to add the second Zac shard. I can't click it, can't drag it... nothing. I'm supposed to be able to reroll any three champion shards, aren't I? I assume this is a bug. If not, Riot should really change the information to clarify that it has to be three different shards. But either way, if someone from Riot could please clarify what the problem is?