Examine this code snippet for checking if a binary tree is balanced:

Examine this code snippet for checking if a binary tree is balanced:

def is_balanced(root):
    def height(node):
        if node is None:
            return 0
        left_height = height(node.left)
        right_height = height(node.right)
        if abs(left_height - right_height) > 1:
            return -1
        return max(left_height, right_height) + 1
    return height(root) != -1

Which issue could cause incorrect results on trees with null nodes?