Can't attack city with 3rd party unit in it

[The_J]

Not sure if the glitch has been posted already (losing overview...; originally posted here):
When you want to attack an enemy city, in which a unit of a friendly 3rd party is stationed, then Civ5 will not allow you to attack the city, instead it asks if you want to declare war on the 3rd party.

Related savegame
There should be somewhere an egyptian city with a persian scout in it, and you can't attack this city.

 

[Petek]

Can't load the save since several mods are present. I don't recall this situation being discussed for Civ 5, but I'm pretty sure it came up in prior versions. Can't remember whether it was considered a bug or "working as intended" though.

 

[DaveGold]

You shouldn't be able to put foreign units into cities at all?