merging shapes into one object before applying a stroke

Hello All,

I've been doing a logo where I letters where converted into object so I could modify their shapes.
I've been tweaking around and I have some letters that overlap, all white, to make it look like a seamless logo word. I'd like to now apply a stroke to it (to have a little simple black contour).

I tried joining the connections between layers but it tells me ; "This selected objects cannot be joined as they are invalid objects (compound paths, closed paths, text object, graphs live paint group).

What am I doing wrong ?

Thanks for reading.


motion pictures + storytelling = ?
-> FILMMAKING !