Why cant I give this armor to a Mound King?

[tinkthank]

In an MP game, I find I cannot give the Monolith armor to a summoned Mound King I want to protect (since he is carrying a Banner of the NL I dont want destroyed).
I can give him all the other items (including armor) in my inventory, but not the Monolith armor. I stripped him naked (no more banner) but cant give him the armor, although I can give him all other armors.
I can give the Monolith armor to other commanders in the same province with body slots, no problem.
Is this a bug?
Thanks
PS. OT Speaking of armor: in the next patch, post-nerf Elemental Armor should no longer read "renders wearer immune to...", but only "offers protection from..."

[Molog]

Monolith armor can't be given to mounted commanders.

[tinkthank]

D'oh -- thanks!