Tree: Depth-First Search - Sum Root to Leaf Numbers
Analyze the following recursive DFS code snippet for summing root-to-leaf numbers. Identify the line causing incorrect sums on some inputs:
```python
def dfs(node, current_sum):
if node is None:
return 0
current_sum = current_sum * 10 + node.val
if node.left is None and node.right is None:
return current_sum
return dfs(node.left, current_sum) + dfs(node.right, current_sum)
```