How to write an in-order traversal to check if its a valid BST but with keeping the previous node as a parameter?

I'm trying to write a recursive code to check if a binary search tree is valid using an in-order traversal but without using an array. In other words, using only a previous value. I'm trying to keep the previous value as a parameter, but the code is failing. I've trie debugging but not sure why the code is not working.

class Solution {
    public boolean isValidBST(TreeNode root) {
        return isValidBSTPrev(root, null);
    }
    public boolean isValidBSTPrev(TreeNode root, TreeNode prev) {
        if(root == null) {
            return true;
        } else if(isValidBSTPrev(root.left, root) 
                  && (prev!=null && root.val > prev.val) 
                  && isValidBSTPrev(root.right, root)) {
            return true;
        } else {
            return false;
        }
    }
}