Examine the following buggy code snippet for summing root-to-leaf numbers using DFS recursion. Which line contains the subtle bug causing incorrect sums on some inputs?

Examine the following buggy code snippet for summing root-to-leaf numbers using DFS recursion. Which line contains the subtle bug causing incorrect sums on some inputs?

class Solution:
    def sumNumbers(self, root: TreeNode) -> int:
        self.total = 0
        def dfs(node, current_number):
            if not node:
                return
            current_number = current_number * 10 + node.val
            self.total += current_number  # Bug here
            if not node.left and not node.right:
                return
            dfs(node.left, current_number)
            dfs(node.right, current_number)
        dfs(root, 0)
        return self.total