Pressing a letter key inside a dropdown not working

Hello,

I have a subscription form in my application with a "Select Country" dropdown.
When you have focus on the dropdown, and press 'U' on keyboard(for example), it brings you to the countries starting by U. etc.

For unknown reason, I can't make it work in Sikuli.
It's a game, so not a standard Windows/Unix application. But it's working when typing manually with the keyboard.
I also have textboxes in that form, and the 'type' function works perfectly.

Tried :
theDropdown.type('U')
type('U') after clicked on the dropdown

keyDown('U')
keyUp('U')

Maybe I'm missing something about pressing letter keys...

Any idea ?
Thanks in advance