Incrementing binary number

[Danielk121]

Hello, I got this assignment of incrementing a binary number (which displays as a string) without using base converting.

i wrote the code which i think is pretty good, but i got stuck with an error message when used this line of code:
return ''.join(lst_binary.insert(0,'1'))
that happens when the prog gets a string like '111', and needs to add another digit at the end of number.

the error i got was

Error:
"TypeError: can only join an iterable"

while was debugging with python-tutor

this is the whole code:

def inc(binary):
    lst_binary = list(binary)
    for bit in range(len(binary)-1, -1, -1):
        if lst_binary[bit] == '0':
            lst_binary[bit] = '1'
            return ''.join(lst_binary)
        else:
            lst_binary[bit] = '0'
    return ''.join(lst_binary.insert(0,'1'))

im not looking though for other ways to do this, just to understand why this line doesnt work