add / in front of each letter

I'm trying to create a recursive function that adds a slash in front of each letter in a string. I know that the base case is " " would return " ". Here are some doc test

add_slash('hello')
'h/e/l/l/o/'
add_slash('')
''

I tried to solve it using list comprehension, but it doesn't add

['/'.join(x) for x in string]

string being a input, but how this code does not add the final / at the end of the string.