Why am I getting '[...]' as a subsequence of my input string?

Question

I am trying to recursively obtain all subsequences for an input string. While I am getting the said subsequences, another element keeps getting added as well. I cannot figure out what is producing this element.

**CODE: **

class Solution:
    
    def subs(self,word,index,cs,arr=[]):
        if(len(word)==index):
            return cs
        else:
                l=word[index]
                arr.append(self.subs(word,index+1,cs,arr))
                arr.append(self.subs(word,index+1,cs+l,arr))
        
        return arr  

    def numMatchingSubseq(self, s: str):
        x=self.subs(s,0,'')
        print(x)
    
x=Solution()
x.numMatchingSubseq('ab')

MY OUTPUT: ['', 'b', [...], 'a', 'ab', [...]]

EXPECTED OUTPUT: ['', 'b', 'a', 'ab']