A path away from the root

A tree has a left subtree of height 3 and no right subtree. Inside that left subtree, two height 2 branches meet. What can this code miss?

int diameter(TreeNode root) {
    if (root == null) return 0;
    return height(root.left) + height(root.right);
}

Assume ordinary supporting code and valid inputs unless the question states otherwise.

Choose an answer before revealing the trace. Keys 1 to 3 work too.