The following code attempts to count nodes in a complete binary tree using the optimal approach. Identify the line containing the subtle bug that can cause incorrect counts for incomplete last levels.

The following code attempts to count nodes in a complete binary tree using the optimal approach. Identify the line containing the subtle bug that can cause incorrect counts for incomplete last levels.

def exists(idx, h, root):
    left, right = 0, 2**(h - 1) - 1
    for _ in range(h - 1):
        mid = (left + right) // 2
        if idx < mid:  # Bug here
            root = root.left
            right = mid
        else:
            root = root.right
            left = mid + 1
    return root is not null