Sacrifice bug?

[ganjatron]

Hey everyone, sorry if this has been answered before, but I've searched through the posts and only found answers to the contrary, so here goes:

I just got sacrifice (awesome spell) and have been adding to my army those units I can no longer find available for purchase. I'm having an issue with one of my favorite units though - Royal Thorn.

For some reason, I can't add more units to this group - is it a bug or working as designed? I have enough leadership (the popup info says "Quantity 3/7") but I can't seem to add any more.

After I select a unit to sacrifice, there is no way to select the Royal Thorn - there is no "glow" underneath it and I don't know what I'm doing wrong. I have over 500 farmers which I'm trying to sacrifice, so I know I have enough units available to create a Royal Thorn. I've already created dozens of Inquisitors, Knights, Royal Snakes, and Horsemen, so I know the procedure... it just won't let me select Royal Thorns as the target.

If this doesn't make sense, I cant try to attach a FRAPS video.