Ok, now I remember where I saw that error before. Do you have the Heroes mod installed? That causes problems for me with this mod.
I think it has something to do with that mod not recognizing new units well, does the same thing with some race mods. Deactivate it, or any other special "options," you might have going.
If that doesn't fix it, upload some screenshots/reports/logs and maybe somebody can figure it out.
(P.S. Boss, that warhound is sweet!)