In the following backtracking code snippet for subsets, which line causes all subsets in the result to be identical?

In the following backtracking code snippet for subsets, which line causes all subsets in the result to be identical?

def subsets(nums):
    res = []
    def backtrack(i, path):
        res.append(path)
        if i == len(nums):
            return
        backtrack(i + 1, path + [nums[i]])
        backtrack(i + 1, path)
    backtrack(0, [])
    return res