Recursivity - Problem

[SupaFlamme]

Hi, I got a problem this code :

def deepconcat(l) :
    if l == [] :
        return None
    elif len(l) == 1 :
        return l[0]
    else :
        for i in l :
            if type(i) == str :
                return l[0] + deepconcat(l[1:])
            if type(i) == list :
                return deepconcat(i+l[1:]) 

I have to turn ["a",["b","6"],"e",["5",["g","h"]],"i"] into "ab6e5ghi".
It works.
If I add a string at the end of the list, for example : ["a",["b","6"],"e",["5",["g","h"]],"i","o"], it works.
But when I add a sub list, for example : ["a",["b","6"],"e",["5",["g","h"]],"i",["k"]], I have a TypeError.
I don't understand why, could you please help me ?