Can't land troops in enemy province

[unmerged(1737)]

Playing as Scotland, at war with Savoy, I can't land my troops in Nice (owned by Savoy).
As I understand it, you're allowed to move troops into the provinces of a country you're at war with - are there any other factors that would impede unloading of troops, or could this be a bug?

 

[unmerged(1522)]

AFAIK, you can't enter a province if your invasion force is too small compared to the enemy garrison.