"Cannot Invade. No Genocide Ships" ??????

So my game auto-patched on Steam, loaded up and found my ongoing save no longer worked. Oops. So I started a new one.

My neighbor declares war, I fight him off, build a couple troop transports and fly off to take his nearby capital planet. Only it won't let me invade.

I try to move my transport to the planet and invade it, same as I have done countless other times in previous games, only I get a bizarre error message.

"CANNOT INVADE. No Genocide Ships"

Huh???? What on Earth does that mean? I have never seen this before, nor can I find any reference to this anywhere on the internet or in this forum. I thought the invasion mechanics were left unchanged, or am I missing something?

I should add I am playing base game with no mods, though with some custom races.