Cannot Terraform Invaded planet?

[Daggath]

Hi Devs,
It seems when I invade a planet own by another race and is something other than Terran, I am unable to change the option to Terraform.
I have the Clean, Maintain and Neglect options only.
After invading I chose to Assimilate them and after it reaches 100% I am still not given the opportunity to make it a Terran world.
Would this be due to the race that is already existing on the planet?

[Lithari]

That is exactly right, once you invade the planet and take it from the previous owner's race matters, this is why i always exterminate them when i conquer a planet.

[Zaimat]

You can terraform the planet if it's beneficial to the native race, since assimilated races are still the ones living on that planet.

eg.: You conquer a Gargal colony (an aquatic race), it will allow you to terraform until Oceanic.