how to use Matlab coder to generate C code with a binary tree

I just want to generate C code where a binary tree is involved. following is the related code, in which I have a structure named "node", which contains members "left" and "right". I want "left" and "right" can also be a "node" structure.

But I always get errors when generating the C code, saying "the Structures don't match". I understand that I need to make sure the new node assigned to left should have the same structure, but don't know how to realize that.

function node=tree()

% initialize the root node

node.val=1;

node.left=struct();

node.right=struct();

% create a new node and assign to left

nkd.val=2;

nkd.left=struct();

nkd.right=struct();

node.left=nkd;