Tree: Depth-First Search - Symmetric Tree (DFS Approach)
Identify the subtle bug in the following code snippet for checking if a binary tree is symmetric:
```python
def isMirror(t1, t2):
if not t1 and not t2:
return True
if not t1 or not t2:
return False
if t1.val != t2.val:
return False
return isMirror(t1.left, t2.left) and isMirror(t1.right, t2.right)
```