Balanced Binary Tree (not working)

/**
 * Example:
 * var ti = TreeNode(5)
 * var v = ti.`val`
 * Definition for a binary tree node.
 * class TreeNode(var `val`: Int) {
 *     var left: TreeNode? = null
 *     var right: TreeNode? = null
 * }
fails: [1,2,2,3,null,null,3,4,null,null,4]
 */
class Solution {

    fun isBalanced(root: TreeNode?): Boolean {
        if(root == null) return true

        val heightLeft = maxHeight(root.left, 1)
        val heightRight = maxHeight(root.right, 1)
        return Math.abs(heightLeft - heightRight) <= 1
    }

    private fun maxHeight(node: TreeNode?, h: Int): Int {
        if(node == null) return h

        return Math.max(
            maxHeight(node.left, h + 1),
            maxHeight(node.right, h + 1)
        )
    }
}